Add RG_88 texture format

Bug: skia:7903
Change-Id: I69b65fc1cfcc2cc5045bb3b75395f9a256ade278
Reviewed-on: https://skia-review.googlesource.com/c/172979
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index e51925e..261c1f9 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -342,6 +342,7 @@
         case kGray_8_as_Red_GrPixelConfig:
         case kRGBA_8888_GrPixelConfig:
         case kRGB_888_GrPixelConfig:
+        case kRG_88_GrPixelConfig:
         case kBGRA_8888_GrPixelConfig:
             return 0;
         case kRGB_565_GrPixelConfig: