Truncate glyph positions to whole device pixels

This addresses servo/servo#11751: The quad shader rounds rect boundaries to
the nearest pixel.  This can cause glyphs' relative positions to change if one
glyph is shifted left while the next is shifted right.  Using floor() ensures
that all glyphs are shifted in the same direction.

src/batch_builder.rs

@@ -436,8 +436,8 @@ impl<'a> BatchBuilder<'a> {
             glyph_key.index = glyph.index;
             let image_info = resource_cache.get_glyph(&glyph_key, frame_id);
             if let Some(image_info) = image_info {
-                let x = glyph.x + image_info.user_data.x0 as f32 / device_pixel_ratio - blur_offset;
-                let y = glyph.y - image_info.user_data.y0 as f32 / device_pixel_ratio - blur_offset;
+                let x = glyph.x.floor() + image_info.user_data.x0 as f32 / device_pixel_ratio - blur_offset;
+                let y = glyph.y.floor() - image_info.user_data.y0 as f32 / device_pixel_ratio - blur_offset;
 
                 let width = image_info.requested_rect.size.width as f32 / device_pixel_ratio;
                 let height = image_info.requested_rect.size.height as f32 / device_pixel_ratio;