Re-adding analytical GeoProc for rectangles

Adapted old GeoProc because its knowledge of the geometry is helpful for computing the distance vector: https://skia.googlesource.com/skia/+/6006d0f8c4f19d19a12de20826f731f52ac822a7/src/gpu/GrAARectRenderer.cpp

Added distance vector capabilities to this GeoProc.

Modified GrDrawContext to used this GeoProc when drawing anti-aliased vectors with a shader that requires distance vectors.

BUG=skia:
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Review-Url: https://codereview.chromium.org/2230513004