Be explicit about CPAL color space (#763)

The OpenType specification defines that this data is sRGB and Skrifa
follows along. Be clear about it here though as even if other things
change with color, this should be sure to be interpreting that data
correctly.

vello/src/scene.rs

@@ -6,7 +6,7 @@ mod bitmap;
 use std::sync::Arc;
 
 use peniko::{
-    color::{palette, DynamicColor, Srgb},
+    color::{palette, AlphaColor, DynamicColor, Srgb},
     kurbo::{Affine, BezPath, Point, Rect, Shape, Stroke, Vec2},
     BlendMode, Blob, Brush, BrushRef, Color, ColorStop, ColorStops, ColorStopsSource, Compose,
     Extend, Fill, Font, Gradient, Image, Mix, StyleRef,
@@ -979,7 +979,9 @@ fn conv_brush(
     }
 }
 
-fn color_index(cpal: &'_ Cpal<'_>, palette_index: u16) -> Option<Color> {
+// The OpenType color palette is defined to be using the sRGB color space.
+// <https://learn.microsoft.com/en-us/typography/opentype/spec/cpal#palette-entries-and-color-records>
+fn color_index(cpal: &'_ Cpal<'_>, palette_index: u16) -> Option<AlphaColor<Srgb>> {
     // The "application determined" foreground color should be used
     // This will be handled by the caller
     if palette_index == 0xFFFF {
@@ -988,7 +990,7 @@ fn color_index(cpal: &'_ Cpal<'_>, palette_index: u16) -> Option<Color> {
     let actual_colors = cpal.color_records_array().unwrap().unwrap();
     // TODO: Error reporting in the `None` case
     let color = actual_colors.get(usize::from(palette_index))?;
-    Some(Color::from_rgba8(
+    Some(AlphaColor::<Srgb>::from_rgba8(
         color.red,
         color.green,
         color.blue,