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/vk/GrVkCaps.cpp b/src/gpu/vk/GrVkCaps.cpp
index 355373b..09798d9 100644
--- a/src/gpu/vk/GrVkCaps.cpp
+++ b/src/gpu/vk/GrVkCaps.cpp
@@ -93,6 +93,7 @@
         case kUnknown_GrPixelConfig:
         case kAlpha_8_as_Alpha_GrPixelConfig:
         case kGray_8_as_Lum_GrPixelConfig:
+        case kAlpha_half_as_Lum_GrPixelConfig:
             SK_ABORT("Unsupported Vulkan pixel config");
             return 0;