Fix Vulkan Copy as draw shader

Bug: skia:7663
Change-Id: I745900ccc56c697e4abb982e921d99bc28502b33
Reviewed-on: https://skia-review.googlesource.com/114687
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
diff --git a/src/gpu/vk/GrVkCopyManager.cpp b/src/gpu/vk/GrVkCopyManager.cpp
index 02d1ca0..f22d99b 100644
--- a/src/gpu/vk/GrVkCopyManager.cpp
+++ b/src/gpu/vk/GrVkCopyManager.cpp
@@ -65,11 +65,10 @@
 
         "layout(set = 1, binding = 0) uniform sampler2D uTextureSampler;"
         "layout(location = 1) in half2 vTexCoord;"
-        "layout(location = 0, index = 0) out half4 fsColorOut;"
 
         "// Copy Program FS\n"
         "void main() {"
-            "fsColorOut = texture(uTextureSampler, vTexCoord);"
+            "sk_FragColor = texture(uTextureSampler, vTexCoord);"
         "}"
     );