In preparation for GL_EXT_multisampled_render_to_texture2.  This makes
the validation reuse what's there for glFramebufferTexture2D in addition
to what GL_EXT_multisampled_render_to_texture requires.

Additionally, it uses packed enums for texture target and id.

Bug: angleproject:4836
Change-Id: Ie778c84e1ff5a0cb6615257f2aff9e04395dd5aa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2332956Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>