fix raster-masking vertex indexing

[mollymerp]

There was a bug in the raster clipping masks with the order of the mask vertices causing rendering artifacts with overlapping triangles which were only noticeable on zooming with translucent raster tiles.

This PR changes to use a SegmentVector with an explicit IndexArray which solves the overlapping triangles when I manually tested the debug page, but I'm not sure the best way to write a unit/rendering test for this.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

[mollymerp]

this very closely matches the native implementation

[mourner]

Perhaps let's add a render test for that too?

[mollymerp]

added a render test – here's the corresponding test on master with the artifacts