vc4: Implement job shuffling

vc4: Implement job shuffling

Track rendering to each FBO independently and flush rendering only when
necessary.  This lets us avoid the overhead of storing and loading the
frame when an application momentarily switches to rendering to some other
texture in order to continue rendering the main scene.

Improves glmark -b desktop:effect=shadow:windows=4 by 27%
Improves glmark -b
    desktop:blur-radius=5:effect=blur:passes=1:separable=true:windows=4
    by 17%

While I haven't tested other apps, this should help X rendering a lot, and
I've heard GLBenchmark needed it too.