Keep dummy/test renderer or is Pixman good enough

[bwidawsk]

With @cmeissl introducing the Pixman renderer (#1228 and #1239), does it make sense to keep the DummyRenderer at all?

Initially my plan was to use DummyRenderer, but it's not as complete as Pixman for use with DrmCompositor. Additionally, back of the envelope runtime testing shows pixman isn't so much slower (in my compositor, worst case is about 10x slower for a test, but still fairly insignificant overall).

I'm not even sure how DummyRenderer would work for real regression testing tbh without adding a fair amount of functionality.