D3D11: Transcode ETC1 to BC1.

Adds a new extension to allow transcode ETC formats to suitable BC formats.
This commit implements ETC1 to BC1. More formats will be supported soon.

BUG=angleproject:1285

Change-Id: Iacbfbc2248dfe1aebf24b92696249a9404e331cd
Reviewed-on: https://chromium-review.googlesource.com/316511
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/validationES.cpp b/src/libANGLE/validationES.cpp
index 0a14d12..12c7612 100644
--- a/src/libANGLE/validationES.cpp
+++ b/src/libANGLE/validationES.cpp
@@ -334,6 +334,7 @@
         case GL_COMPRESSED_RGBA_S3TC_DXT1_EXT:
         case GL_COMPRESSED_RGBA_S3TC_DXT3_ANGLE:
         case GL_COMPRESSED_RGBA_S3TC_DXT5_ANGLE:
+        case GL_ETC1_RGB8_LOSSY_DECODE_ANGLE:
             return true;
 
         default: