Only fill rows needed for transparency at bottom of sixels.

Little optimization, to not waste cycles; only the bottom sliver that
is longer than the original frame buffer needs to be transparency
filled. The other data is filled from the original framebuffer.

src/framebuffer.cc

@@ -106,14 +106,14 @@ uint8_t **Framebuffer::row_data() {
 
 void Framebuffer::AlphaComposeBackground(const bgcolor_query &get_bg,
                                          rgba_t pattern_col, int pwidth,
-                                         int pheight) {
+                                         int pheight, int start_row) {
     if (!get_bg) return;  // -b none
 
-    iterator pos = begin();
+    iterator pos = begin() + (start_row * width());
     for (/**/; pos < end(); ++pos) {
         if (pos->a < 0xff) break;  // First pixel that is transparent.
     }
-    if (pos == end()) return;  // Nothing transparent all the way to the end.
+    if (pos >= end()) return;  // Nothing transparent all the way to the end.
 
     // Need to do alpha blending, so only now we have to retrieve the bgcolor.
     const rgba_t bgcolor = get_bg();

src/framebuffer.h

@@ -97,10 +97,14 @@ class Framebuffer {
     // pixel will be the pattern color. That creates a 'checkerboard-pattern'
     // sometimes used to display transparent pictures.
     //
+    // "start_row" is to start the transparency filling further down; it is
+    // only really needed as an optimization in the sixels canvas.
+    //
     // This Alpha compositing merges in the linearized colors domain.
     using bgcolor_query = std::function<rgba_t()>;
     void AlphaComposeBackground(const bgcolor_query &get_bg, rgba_t pattern,
-                                int pattern_width, int pattern_height);
+                                int pattern_width, int pattern_height,
+                                int start_row = 0);
 
     // The raw internal buffer containing width()*height() pixels organized
     // from top left to bottom right.

src/sixel-canvas.cc

@@ -103,7 +103,8 @@ void SixelCanvas::Send(int x, int dy, const Framebuffer &fb_orig,
     fb->AlphaComposeBackground(options_.bgcolor_getter,
                                options_.bg_pattern_color,
                                options_.pattern_size * options_.cell_x_px,
-                               options_.pattern_size * options_.cell_y_px / 2);
+                               options_.pattern_size * options_.cell_y_px / 2,
+                               fb_orig.height());
     // .. overwrite with whatever is in the orig.
     std::copy(fb_orig.begin(), fb_orig.end(), fb->begin());