fixup! Test that fast color clears work properly after a blit.
fixup! Test reading from a buffer after fast clearing it.
Add manual test that SwapBuffers properly resolves fast clears.

Unfortunately, this test can't be run in an automated fasion, because
any attempt to read the front or back buffer using glReadPixels()
would force a resolve, nullifying the test.  So this test isn't added
to all.tests.  But it's still useful to keep around for manually
testing fast color clear functionality.