sample-mask-execution: fix misleading description

This has never rendered purple. Working correctly, it renders
half-intensity yellow.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
sample-mask-execution: resolve into fbo always

If the MSAA resolve happens into the default framebuffer, we don't get
the colors we expect.

Adjusts the test to resolve into an FBO first, and then blit the result
to either the test FBO or the default framebuffer.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>