It is legal to blit between GL_RGBA8 and GL_BGRA8 sources and
destinations when resolving multisampled renderbuffers. Expand
BlitFramebuffer's validation to handle this case.

Query GL_MAX_VERTEX_OUTPUT_COMPONENTS on the Core Profile to work
around an error generated on macOS when querying
GL_MAX_VARYING_COMPONENTS.

Expand the BlitFramebuffer tests to cover these cases and start
running them on the OpenGL backend.

Fix detectition of multisampled D3D11 rendertargets when the sample count
is 1.

BUG=angleproject:891

Change-Id: Ief5531756651caa66f612e647d3d5c05c8c51ff5
Reviewed-on: https://chromium-review.googlesource.com/587459Reviewed-by: Kenneth Russell <kbr@chromium.org>