Bug 1512010 - add wrench test for text snapping in raster spaces. r=gw

gfx/wr/wrench/reftests/text/raster-space-snap-ref.yaml

@@ -0,0 +1,10 @@
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 480, 80]
+      raster-space: screen
+      items:
+      - text: "a Bcd Efgh Ijklm Nopqrs Tuvwxyz"
+        origin: 20.5 50
+        size: 20
+        font: "FreeSans.ttf"

gfx/wr/wrench/reftests/text/raster-space-snap.yaml

@@ -0,0 +1,10 @@
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 480, 80]
+      raster-space: local(1.0)
+      items:
+      - text: "a Bcd Efgh Ijklm Nopqrs Tuvwxyz"
+        origin: 20.5 50
+        size: 20
+        font: "FreeSans.ttf"

gfx/wr/wrench/reftests/text/reftest.list

@@ -68,3 +68,4 @@ fuzzy(1,113) platform(linux) == raster-space.yaml raster-space.png
 == shadow-image.yaml shadow-solid-ref.yaml
 options(disable-aa) == snap-clip.yaml snap-clip-ref.yaml
 platform(linux) == perspective-clip.yaml perspective-clip.png
+options(disable-subpixel) == raster-space-snap.yaml raster-space-snap-ref.yaml