Move non-overridable functions onto GrGLShaderBuilder

Also moves swizzle & modulation computation / storage.
Now everything on GrGLProgramStage is virtual with no (or an empty) default implementation.

Open questions:
  Should the kMulByAlphaMask fetches be a GrCustomStage? Or should we glom those onto emitDefaultFetch()? I assume the former.

Next steps: mappings, then texture domains & choosing texture functions.

[bsalomon, 2012-05-24 17:40:00]

On 2012/05/24 17:03:24, TomH wrote:
LGTM

I think it is ok for mul by alpha to be its own stage. It's only used in
isolation today. In the future when zero-texture stages are possible, then it
would be trivial to concat it onto any effect if we decide that's important.

[TomH, 2012-05-24 17:54:26]

Committed with r4045.