Ensure WebGLCompatibilityTest.EnablePixelBufferObjectExtensions functions
in surfaceless mode by creating a framebuffer to do the test glReadPixels
from.

BUG=angleproject:4079

Change-Id: I109b3cdeb85625a6a97832ecf8130d53a4074af9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1893058Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>