[Impeller] Create a fixed-stop gradient variant for SSBOs

[jonahwilliams]

In order to support arbitrary stops, the gradient fragment shaders interate over the stop data (see flutter/engine#39174). This is more expensive than it needs to be in cases where we know the stops are evenly spaced - the exact colors can be looked up and lerp'd. We can quickly compute this in the entity contents and set a uniform value and branch, but today that will likely lead to most shaders executing both branches.

This should be done with shader variants following #119357

[jonahwilliams]

Despite looking at a lot of different apps with large gradients, this has never been a problem. probably not worth a specific variant.

[github-actions]

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