Implement correct handling of the overflow property

SVG has a default user-agent stylesheet that makes some elements
`overflow: hidden`, deviating from the default overflow property value
of `visible`.

Previously, this behaviour had been roughly hacked in but some instances
weren't working.  For example, <svg> elements ignored overflow being set
to visible.

Now overflow is correctly handled as per spec.

Fixes #164

lib/prawn/svg/css/stylesheets.rb

@@ -1,5 +1,7 @@
 module Prawn::SVG::CSS
   class Stylesheets
+    USER_AGENT_STYLESHEET = 'svg, symbol, image, marker, pattern, foreignObject { overflow: hidden }'.freeze
+
     attr_reader :css_parser, :root, :media
 
     def initialize(css_parser, root, media = :all)
@@ -9,13 +11,18 @@ def initialize(css_parser, root, media = :all)
     end
 
     def load
+      load_user_agent_stylesheet
       load_style_elements
       xpath_styles = gather_xpath_styles
       associate_xpath_styles_with_elements(xpath_styles)
     end
 
     private
 
+    def load_user_agent_stylesheet
+      css_parser.add_block!(USER_AGENT_STYLESHEET)
+    end
+
     def load_style_elements
       REXML::XPath.match(root, '//style').each do |source|
         data = source.texts.map(&:value).join

lib/prawn/svg/elements/base.rb

@@ -270,4 +270,14 @@ def extract_element_from_url_id_reference(value, expected_type = nil)
   def href_attribute
     attributes['xlink:href'] || attributes['href']
   end
+
+  def overflow_hidden?
+    ['hidden', 'scroll'].include?(computed_properties.overflow)
+  end
+
+  def clone_element_source(source)
+    new_source = source.dup
+    document.element_styles[new_source] = document.element_styles[source]
+    new_source
+  end
 end

lib/prawn/svg/elements/marker.rb

@@ -46,8 +46,7 @@ def apply_marker(element, point: nil, angle: 0)
 
       element.add_call 'transformation_matrix', 1, 0, 0, 1, -ref_x * sizing.x_scale, ref_y * sizing.y_scale
 
-      # `overflow: visible` must be on the <marker> element
-      if properties.overflow != 'visible'
+      if overflow_hidden?
         point = [sizing.x_offset * sizing.x_scale, y(sizing.y_offset * sizing.y_scale)]
         element.add_call "rectangle", point, sizing.output_width, sizing.output_height
         element.add_call "clip"

lib/prawn/svg/elements/use.rb

@@ -53,7 +53,7 @@ def apply
   def process_child_elements
     add_call 'save'
 
-    source = referenced_element_source.dup
+    source = clone_element_source(referenced_element_source)
 
     if referenced_element_class == Prawn::SVG::Elements::Viewport
       source.attributes['width'] = @width || '100%'

lib/prawn/svg/elements/viewport.rb

@@ -16,8 +16,11 @@ def apply
       add_call 'transformation_matrix', 1, 0, 0, 1, @x, -@y
     end
 
-    add_call 'rectangle', [0, y(0)], @sizing.output_width, @sizing.output_height
-    add_call 'clip'
+    if overflow_hidden?
+      add_call 'rectangle', [0, y(0)], @sizing.output_width, @sizing.output_height
+      add_call 'clip'
+    end
+
     add_call 'transformation_matrix', @sizing.x_scale, 0, 0, @sizing.y_scale, 0, 0
     add_call 'transformation_matrix', 1, 0, 0, 1, -@sizing.x_offset, @sizing.y_offset
   end

spec/prawn/svg/css/stylesheets_spec.rb

@@ -62,6 +62,7 @@
       width_and_styles = result.map { |k, v| [k.attributes["width"].to_i, v] }.sort_by(&:first)
 
       expected = [
+        [0, [["overflow", "hidden", false]]],
         [1, [["fill", "#ff0000", false]]],
         [2, [["fill", "#ff0000", false], ["fill", "#330000", false], ["fill", "#440000", false], ["fill", "#220000", false]]],
         [3, [["fill", "#ff0000", false], ["fill", "#00ff00", false]]],
@@ -114,6 +115,7 @@
     it "scans the document for style tags and adds the style information to the css parser" do
       css_parser = instance_double(CssParser::Parser)
 
+      expect(css_parser).to receive(:add_block!).with("svg, symbol, image, marker, pattern, foreignObject { overflow: hidden }")
       expect(css_parser).to receive(:add_block!).with("a\n  before>\n  x  y\n  inside <>&gt;\n  k  j\n  after\nz")
       expect(css_parser).to receive(:add_block!).with("hello")
       allow(css_parser).to receive(:each_rule_set)

spec/prawn/svg/elements/marker_spec.rb

@@ -18,18 +18,19 @@
   end
 
   let(:document) { Prawn::SVG::Document.new(svg, [800, 600], {width: 800, height: 600}) }
-  let(:state) { Prawn::SVG::State.new }
 
-  let(:line_element) do
-    Prawn::SVG::Elements::Line.new(document, document.root.elements[2], [], state)
+  def new_state
+    state = Prawn::SVG::State.new
+    state.viewport_sizing = document.sizing
+    state
   end
 
-  subject do
-    Prawn::SVG::Elements::Marker.new(document, document.root.elements[1], [], state)
+  let(:line_element) do
+    Prawn::SVG::Elements::Line.new(document, document.root.elements[2], [], new_state)
   end
 
-  before do
-    state.viewport_sizing = document.sizing
+  subject do
+    Prawn::SVG::Elements::Marker.new(document, document.root.elements[1], [], new_state)
   end
 
   describe "#parse" do
@@ -70,8 +71,8 @@
             ["clip", [], {}, []],
             ["transformation_matrix", [0.3, 0, 0, 0.3, 0, 0], {}, []],
             ["transparent", [1.0, 1.0], {}, [
-                ["stroke_color", ["000000"], {}, []],
-                ["line_width", [100.0], {}, []],
+                ["fill_color", ["000000"], {}, []],
+                ["line_width", [1.0], {}, []],
                 ["cap_style", [:butt], {}, []],
                 ["join_style", [:miter], {}, []],
                 ["undash", [], {}, []],