Add more premultiplied gradient reftests.

The existing gradient tests don't have any coverage for whether
the input color is correctly pre-multiplied.

Additionally, the angle test wasn't actually triggering the angle
gradient shader, and was running the aligned shader.

Author: gw3583

wrench/reftests/gradient/premultiplied-aligned-2.png

@@ -0,0 +1,8 @@
+---
+root:
+  items:
+    - type: gradient
+      bounds: 50 50 200 200
+      start: 0 100
+      end: 200 100
+      stops: [0.0, [255.0, 0.0, 0.0, 0.5], 0.5, [0.0, 255.0, 0.0, 0.5], 1.0, [0.0, 0.0, 255.0, 0.5]]

wrench/reftests/gradient/premultiplied-aligned-2.yaml

@@ -0,0 +1,8 @@
+---
+root:
+  items:
+    - type: gradient
+      bounds: 50 50 200 200
+      start: 0 0
+      end: 200 200
+      stops: [0.0, [255.0, 0.0, 0.0, 0.5], 0.5, [0.0, 255.0, 0.0, 0.5], 1.0, [0.0, 0.0, 255.0, 0.5]]

wrench/reftests/gradient/premultiplied-angle-2.png

@@ -3,8 +3,6 @@ root:
   items:
     - type: gradient
       bounds: 50 50 200 200
-      # offset the start and end points slightly so
-      # we force the angle gradient shader
-      start: 0 100
-      end: 200 100.000001
+      start: 0 0
+      end: 200 200
       stops: [0.0, red, 0.5, [0.0, 0.0, 0.0, 0.0], 1.0, green]

wrench/reftests/gradient/premultiplied-angle-2.yaml

@@ -0,0 +1,8 @@
+---
+root:
+  items:
+    - type: radial-gradient
+      bounds: 50 50 200 200
+      center: 100 100
+      radius: 100 100
+      stops: [0.0, [255.0, 0.0, 0.0, 0.5], 0.5, [0.0, 255.0, 0.0, 0.5], 1.0, [0.0, 0.0, 255.0, 0.5]]

wrench/reftests/gradient/premultiplied-angle.png

@@ -2,6 +2,10 @@
 == premultiplied-angle.yaml premultiplied-angle.png
 == premultiplied-radial.yaml premultiplied-radial.png
 
+== premultiplied-aligned-2.yaml premultiplied-aligned-2.png
+== premultiplied-angle-2.yaml premultiplied-angle-2.png
+== premultiplied-radial-2.yaml premultiplied-radial-2.png
+
 == linear.yaml linear-ref.png
 == linear-reverse.yaml linear-ref.png
 fuzzy(1,35000) == linear-stops.yaml linear-stops-ref.png