[Impeller] Switch to uniform arrays for gradient data on non-SSBO hardware

[flar]

Currently the most generalized form of the Impeller gradient shaders uses an SSBO to store the gradient information, but SSBO data is not supported on older platforms. To make the capability more general we introduce variants of the gradient shaders that uses uniform arrays which are more widely supported.

[flutter-dashboard]

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Changes reported for pull request #56441 at sha 46ae8aa

[flutter-dashboard]

Golden file changes are available for triage from new commit, Click here to view.

Changes reported for pull request #56441 at sha b99aa78

[flar]

The goldens all make sense to me:

• The 12 new images from the new test I added
• All the rest of the tests are restricted to the OpenGLES platform:
  • A whole bunch of off-by-1-color-component
  • A couple of off-by-3ish color component cases
  • A couple of off-by-1-pixel on the edges of some sweep gradients
  • One case of an actual bug fix for the overlapping stops test case now handled correctly by the new shader