Fixes https://github.com/flutter/flutter/issues/133325

Due to the number of uniforms present, the ink_sparkle shader can't run without post processing (on all target platforms) that the impellerc offline compiler doesn't perform. However, we can't just move the uniforms into a uniform buffer object (UBO) because the Skia backend doesn't support it.

Rather than work around this in the compiler, we can reduce the uniform count by 1) packing four floats into a single vec4 2) removing a uniform for what is effectively a constant.

This should have no visible effects, and if any scubas fail it means I did this wrong 😆