ANGLE_framebuffer_blit is already exposed but has restrictions on
scaling/flipping and mismatched color buffer formats. When
NV_framebuffer_blit, EXT_framebuffer_blit, or an ES3 context is
available we can expose a less restrictive BlitFramebuffer.

Bug: angleproject:5474
Bug: chromium:1157057
Change-Id: I916ee39c8d6120216f91461080eb3ee9ca777e29
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2601165
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>