Fix issue where GrQuadPerEdgeAA GP doesn't emit coord transforms

This happens when all the FPs are sampled with explicit coords.

The GrGLSLGeometryProcessor::emitTransforms() helper required a valid VS
var for local coords even when all FPs use explicit coords and wouldn't
use a local coords var. It now only requires the variable be valid if
there is an FP with a coord transform that is not explicitly sampled.

Change-Id: I8008e854c738a17b207c82bb791e230ef4d5364d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/272459
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed