add emoji family for emoji clusters (#56)

Quick and dirty fallback for emoji. This can be done more efficiently
(and should be handled in fontique directly) but this should get us
functioning emoji mapping for capable renderers.

src/shape.rs

@@ -10,6 +10,8 @@ use super::style::{Brush, FontFeature, FontVariation};
 use crate::util::nearly_eq;
 #[cfg(feature = "std")]
 use crate::Font;
+#[cfg(feature = "std")]
+use fontique::QueryFamily;
 use fontique::{self, Attributes, Query, QueryFont};
 use swash::shape::*;
 #[cfg(feature = "std")]
@@ -212,7 +214,8 @@ impl<'a, 'b, B: Brush> partition::Selector for FontSelector<'a, 'b, B> {
 
     fn select_font(&mut self, cluster: &mut CharCluster) -> Option<Self::SelectedFont> {
         let style_index = cluster.user_data() as u16;
-        if style_index != self.style_index {
+        let is_emoji = cluster.info().is_emoji();
+        if style_index != self.style_index || is_emoji || self.fonts_id.is_none() {
             self.style_index = style_index;
             let style = &self.styles[style_index as usize].style;
             let fonts_id = style.font_stack.id();
@@ -223,7 +226,15 @@ impl<'a, 'b, B: Brush> partition::Selector for FontSelector<'a, 'b, B> {
             };
             let variations = self.rcx.variations(style.font_variations).unwrap_or(&[]);
             let features = self.rcx.features(style.font_features).unwrap_or(&[]);
-            if self.fonts_id != Some(fonts_id) {
+            if is_emoji {
+                let fonts = self.rcx.stack(style.font_stack).unwrap_or(&[]);
+                let fonts = fonts.iter().map(|id| QueryFamily::Id(*id));
+                self.query
+                    .set_families(fonts.chain(core::iter::once(QueryFamily::Generic(
+                        fontique::GenericFamily::Emoji,
+                    ))));
+                self.fonts_id = None;
+            } else if self.fonts_id != Some(fonts_id) {
                 let fonts = self.rcx.stack(style.font_stack).unwrap_or(&[]);
                 self.query.set_families(fonts.iter().copied());
                 self.fonts_id = Some(fonts_id);