Bug 1529260 - Match shader and CPU glyph rasterization logic for glyp…

…hs in local space. r=lsalzman

This patch makes the CPU side incorporate the raster scale when
calculating the subpixel position of a glyph. It also makes the shader
side not include the glyph scale factor when recalculating the glyph
position (since it was not known/used when determining the subpixel
position in the first place). This makes the subpixel position stable
when we transition between Screen and Local(raster_scale) spaces.

Differential Revision: https://phabricator.services.mozilla.com/D62812

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/9bd4120bc9779fe1c42fb45246044a23f2e95d25

webrender/res/ps_text_run.glsl

@@ -186,9 +186,11 @@ void main(void) {
 #else
     float raster_scale = float(ph.user_data.z) / 65535.0;
 
+    // Scale in which the glyph is snapped when rasterized.
+    float glyph_raster_scale = raster_scale * task.device_pixel_scale;
+
     // Scale from glyph space to local space.
-    float glyph_scale_inv = res.scale / (raster_scale * task.device_pixel_scale);
-    float glyph_scale = 1.0 / glyph_scale_inv;
+    float glyph_scale_inv = res.scale / glyph_raster_scale;
 
     // Glyph raster pixels do not include the impact of the transform. Instead it was
     // replaced with an identity transform during glyph rasterization. As such only the
@@ -204,7 +206,7 @@ void main(void) {
     // - The transform has perspective or does not have a 2d inverse (Screen or local space).
     // - The transform's scale will result in result in very large rasterized glyphs and
     //   we clamped the size. This will imply local raster space.
-    vec2 raster_glyph_offset = floor(glyph.offset * glyph_scale + snap_bias);
+    vec2 raster_glyph_offset = floor(glyph.offset * glyph_raster_scale + snap_bias) / res.scale;
 
     // Compute the glyph rect in local space.
     //

webrender/src/prim_store/text_run.rs

@@ -361,10 +361,15 @@ impl TextRunPrimitive {
         if self.glyph_keys_range.is_empty() || cache_dirty {
             let subpx_dir = self.used_font.get_subpx_dir();
 
+            let transform = match self.raster_space {
+                RasterSpace::Local(scale) => FontTransform::new(scale, 0.0, 0.0, scale),
+                RasterSpace::Screen => self.used_font.transform,
+            };
+
             self.glyph_keys_range = scratch.glyph_keys.extend(
                 glyphs.iter().map(|src| {
                     let src_point = src.point + prim_offset;
-                    let world_offset = self.used_font.transform.transform(&src_point);
+                    let world_offset = transform.transform(&src_point);
                     let device_offset = surface.device_pixel_scale.transform_point(world_offset);
                     GlyphKey::new(src.index, device_offset, subpx_dir)
                 }));