Allow .fp files to return a color from main().

Existing .fp files that assign their output to sk_OutColor will continue
to work as before. An error will be generated if an .fp file tries to
mix-and-match return statements and sk_OutColor assignment; only one or
the other is allowed within a single .fp file.

Change-Id: I53bc0e8b79b2fdd64a020a7e5b35a905f9b05c94
Bug: skia:10549
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313197
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
diff --git a/src/gpu/effects/generated/GrRectBlurEffect.cpp b/src/gpu/effects/generated/GrRectBlurEffect.cpp
index 6e98b02..a0acd0c 100644
--- a/src/gpu/effects/generated/GrRectBlurEffect.cpp
+++ b/src/gpu/effects/generated/GrRectBlurEffect.cpp
@@ -143,6 +143,7 @@
     if (isFast != that.isFast) return false;
     return true;
 }
+bool GrRectBlurEffect::usesExplicitReturn() const { return false; }
 GrRectBlurEffect::GrRectBlurEffect(const GrRectBlurEffect& src)
         : INHERITED(kGrRectBlurEffect_ClassID, src.optimizationFlags())
         , rect(src.rect)