[Impeller] Float samplers can get re-ordered compared to SkSL

[jonahwilliams]

When i looked at this last I thought that this was handled correctly via our uniform mapping, but @zanderso has shared a few places where that isn't the case. Need to investigate what is different about these and whether or not we should revist uniform re-mapping for floats.

[zanderso]

I believe there's still a dependence on use order. @jonahwilliams it shows up in the glowing text example in Impeller on Metal that I sent you awhile back. Maybe it's only cropped up after flutter/engine#39147 and flutter/engine#39198 --- I haven't bisected yet.

[jonahwilliams]

Those PRs only adjusted sampler use order - are you seeing float re-ordering? I'd expect them to be re-ordered but the mapping data should account for it. We don't correctly handle uniforms getting optimized out yet.

[zanderso]

Hmm. Shoot. This isn't repro'ing for me anymore. Sorry for the false alarm. I think I probably wasn't being sufficiently paranoid when swapping among a few different framework and engine versions...