Fix bug in COLRv1 glyph splitting, layers start_idx=0, and ot-rounding#1768
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`Colrv1RunType` only stored gradient colors when determining if consecutive shapes should be grouped into the same run. Two shapes with identical colors but different gradient geometry (e.g., different angles for linear gradients) were incorrectly treated as the same run and merged into a single `.colorN` glyph. Fixes #1766
When building `PaintColrLayers`, the code computed `start_idx` from `layer_list.num_layers`, which was always 0 during processing. The field was only set once at the very end of all processing. This caused all glyphs to incorrectly reference layers starting at index 0, making them share/overwrite each other's layers. Fixes #1767
This fixes two issues with gradient coordinate rounding: 1. Compound rounding error: When calculating perpendicular points for linear gradients (when p2 is not explicitly provided), the code rounded p0 and p1 to i16 first, then calculated the perpendicular from the rounded values. This caused compound rounding errors. 2. Wrong rounding mode: Used Rust's `.round()` (round half away from zero) instead of fontTools' `otRound` (round half up per OpenType spec).
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Fixes #1766 and #1767
plus, use OtRound to round gradient's geometry as fontTools does
we now match
ttx-diff resources/testdata/glyphs3/COLRv1-manyshapes-per-glyph.glyphswith "output is indentical"