mesa: Make renderbuffer FBO attachments not layered

mesa: Make renderbuffer FBO attachments not layered

For framebuffer completeness checks, consider renderbuffers as not
layered. Previously, they would have counted as layered if a layered
textured had previously been bound to the same attachment point. This
could cause framebuffer completeness checks to incorrectly fail with
GL_FRAMEBUFFER_INCOMPLETE_LAYER_TARGETS, even if no layered attachments
were present.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89026