Add kRGB_888X_GrPixelConfig

This is needed to support importing of RGBX AHB into skia's vulkan backend.
With this CL we only enable the new pixel config to be textureable.

Bug: skia:
Change-Id: Iba9180c14f3ef633ae846091cf453d68f82ce544
Reviewed-on: https://skia-review.googlesource.com/c/192035
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index dd18074..5994657 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -341,6 +341,7 @@
         case kGray_8_as_Red_GrPixelConfig:
         case kRGBA_8888_GrPixelConfig:
         case kRGB_888_GrPixelConfig:
+        case kRGB_888X_GrPixelConfig:
         case kRG_88_GrPixelConfig:
         case kBGRA_8888_GrPixelConfig:
             return 0;