GL_BACK, GL_DEPTH, and GL_STENCIL are only valid for the default
framebuffer, while GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT,
GL_DEPTH_STENCIL_ATTACHMENT, and GL_COLOR_ATTACHMENTi are only valid
for non-default framebuffer objects.

Also implement the color encoding query.

Change-Id: I153ae9407850a30ed14d9ae145ee3504ba71029a
Reviewed-on: https://swiftshader-review.googlesource.com/14569Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>