Emulate RGB10 (no alpha) on desktop OpenGL. The OpenGL ES extension GL_EXT_texture_type_2_10_10_10_REV requires RGB and RGBA formats to be supported but Desktop OpenGL does not support RGB. Emulate it with the existing emulatedAlphaChannel path in TextureGL. Bug: chromium:1300575 Change-Id: I0e6d1044afbc55cf6d4f987cc98b357ff11ff006 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3517346 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>