Start of rewrite of GrFragmentProcessor optimizations.

This adds a replacement for computeInvariantOutput buts does not use it yet. The replacement allows for three types of optimizations:

* known input color -> known output color for GrFP elimination
* tracking of whether all color processors modulate their input for the "tweak alpha" optimziation
* opaqueness tracking

This loses some of the generality of computInvariantOutput. It does not track the known output status of individual color components (other than opaque alpha). It does not track whether GrFragmentProcessors read their input color. It doesn't allow a processor that will receive non-constant output to advertise that it produces a constant output. These could probably be added back in the unlikely case that they prove valuable.

Unlike computeInvariantOutput the optimizations are decided at instantiation time and constant colors are expressed as GrColor4f rather than GrColor.

Change-Id: I684d3f9050693dde2d28154fa695e049ed8cf61a
Reviewed-on: https://skia-review.googlesource.com/7481
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

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