This lets the trace perf tests run configurable number of frames
within a single swap. The offscreen config is similar to how
gfxbench works. It renders to a user FBO (by overriding calls to
BindFramebuffer) and then composits multiple frames into the real
backbuffer. This allows us to get a perf measurement with less
overhead from composition and display.

Adds emulation for some APIs that operate on Framebuffers like
BindFramebuffer, Invalidate, DrawBuffers and ReadBuffer.

Bug: angleproject:4845
Change-Id: I1044c1d52c82f1c215a68a6c46d74c52ed0f3d2a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2300207
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>