mesa: Enable EXT_compressed_ETC1_RGB8_sub_texture

mesa: Enable EXT_compressed_ETC1_RGB8_sub_texture

Since we already have the functionality in place and games
like Game of Thrones seem to depend on this extension, I
think it makes sense to enable it by making it part of
the extension string even though it's still a draft:

https://www.khronos.org/registry/gles/extensions/EXT/EXT_compressed_ETC1_RGB8_sub_texture.txt

Note: OES_compressed_ETC1_RGB8_sub_texture seems to be listed
in gl2ext.h, but there's no documentation for it in the KHR
registry

Signed-off-by: Plamena Manolova <plamena.manolova@intel.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>