[Impeller] consolidate transforms in PositionUVWriter

[flar]

Consolidates the 3 coordinate operations in the PositionUVWriter into a single transform for efficiency.

[flar]

Notable benchmark results, measured locally. Basically, other than the Round cases which currently involve a lot of cubic tessellation overhead, the gains are in the realm of 20-30%.

------------------------------------------------------------------------------------------
Benchmark                                                Current        New       New/Cur
------------------------------------------------------------------------------------------
BM_StrokePolyline/stroke_Cubic_Butt_Bevel_uv             9052 ns      6357 ns      70.2%
BM_StrokePolyline/stroke_Cubic_Butt_Miter_uv             8972 ns      7522 ns      83.8%
BM_StrokePolyline/stroke_Cubic_Butt_Round_uv            18183 ns     17550 ns      96.5%
BM_StrokePolyline/stroke_Cubic_Square_Bevel_uv           9103 ns      6521 ns      71.6%
BM_StrokePolyline/stroke_Cubic_Round_Bevel_uv            9175 ns      6378 ns      69.5%
BM_StrokePolyline/stroke_Quadratic_Butt_Bevel_uv         9015 ns      6382 ns      70.8%
BM_StrokePolyline/stroke_Quadratic_Butt_Miter_uv         8610 ns      7212 ns      83.8%
BM_StrokePolyline/stroke_Quadratic_Butt_Round_uv        17958 ns     17601 ns      98.0%
BM_StrokePolyline/stroke_Quadratic_Square_Bevel_uv       8922 ns      6252 ns      70.1%
BM_StrokePolyline/stroke_Quadratic_Round_Bevel_uv        8859 ns      6353 ns      71.7%
BM_StrokePolyline/stroke_RRect_Butt_Bevel_uv             1419 ns      1093 ns      77.0%

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Changes reported for pull request #50635 at sha 8891105

[flar]

The golden failure notice is stale, fixed by the second commit...