Combined stroke/fill shader

The previous attempt to render outlines in the fill shader lead to rendering artefacts due to numerical instability. It was also mathematically incorrect. That approach may still be valid for very thin/cosmetic pen widths.

Another option is adding fill functionality to the stroke shader, possibly combined with a pure non-AA triangle fill shader in a separate draw call.

We would need to do performance testing on various hardware to determine if any of these strategies are worthwhile.