Implement anti-aliased DrawPath for MetalSDF

We should have a design doc that outlines how we are going to implement DrawPath with anti-aliasing. Some ideas are:
1) using Inigo Quilez's SDF algorithms for paths -- It might get complicated when combing bezier with straight lines.
1) using @flar's approach of inflating meshes and burning in the alpha as vertex attribs

attn: @walley892

No files, tests, or entry points are named. Start with the issue discussion and compare the proposed Inigo Quilez SDF and mesh-inflation approaches for anti-aliased DrawPath in MetalSDF; done means a design document that explains the chosen approach and how it handles Bézier curves and straight lines.

Written by the indexing model from the issue text.