Revert "Revert "remove 'in uniform' support from GrSkSLFP, make rules more clear""

This reverts commit 85705c1b3bc53d887fddec1912e9c155044a5b2a.

Change-Id: If189dafce53491728296a4292c76af55b05835ac
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244509
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index ca1ee23..c337509 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -46,7 +46,7 @@
 
 GR_FP_SRC_STRING SKSL_DITHER_SRC = R"(
 // This controls the range of values added to color channels
-layout(key) in int rangeType;
+in int rangeType;
 
 void main(float x, float y, inout half4 color) {
     half value;