Reland "SkSL is now pickier about type conversions"

This is a reland of 91c1d08bc391c997418512b9599f3acd3b2c985d

Original change's description:
> SkSL is now pickier about type conversions
> 
> Bug: skia:
> Change-Id: I4e8b8f229f4e4344f160b0dbb41832764d0b75bd
> Reviewed-on: https://skia-review.googlesource.com/c/188311
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>

Bug: skia:
Change-Id: I727cad061afc0a5ee6f4d2df789330d809dd110a
Reviewed-on: https://skia-review.googlesource.com/c/189643
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/gpu/vk/GrVkCopyManager.cpp b/src/gpu/vk/GrVkCopyManager.cpp
index a41d2b6..6ecd6ba 100644
--- a/src/gpu/vk/GrVkCopyManager.cpp
+++ b/src/gpu/vk/GrVkCopyManager.cpp
@@ -55,7 +55,7 @@
 
         "// Copy Program VS\n"
         "void main() {"
-            "vTexCoord = inPosition * uTexCoordXform.xy + uTexCoordXform.zw;"
+            "vTexCoord = half2(inPosition * uTexCoordXform.xy + uTexCoordXform.zw);"
             "sk_Position.xy = inPosition * uPosXform.xy + uPosXform.zw;"
             "sk_Position.zw = half2(0, 1);"
         "}"