Add Ganesh support for LUM16F (take 2)

When Chrome has a LUM16F texture they tell Skia it is R16F. Although this has been working for them so far it causes trouble with some upcoming changes.

Change-Id: I2473f70e4f725128f143c2dfb08adb79f3c7c166
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228565
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/GrDataUtils.cpp b/src/gpu/GrDataUtils.cpp
index 0ac04f8..939ac3b 100644
--- a/src/gpu/GrDataUtils.cpp
+++ b/src/gpu/GrDataUtils.cpp
@@ -235,6 +235,7 @@
             }
             break;
         }
+        case kAlpha_half_as_Lum_GrPixelConfig:                  // fall through
         case kAlpha_half_as_Red_GrPixelConfig:                  // fall through
         case kAlpha_half_GrPixelConfig: {
             SkHalf alphaHalf = SkFloatToHalf(colorf.fA);