Allow Tessellator to operate on provided GrQuads

To facilitate this, the GrQuadBuffer::Iter's local GrQuads that are
modified on each next() are now allowed to be operated on for the AA
inset/outsetting. Previously this required additional GrQuads on the
stack to hold the results, and additional guards for accessing localQuad()
when the entry didn't have actual coords.

With this change, a 2D op should have its device and src GrQuads' Ws
set to 1 once, and then they are completely ignored for all iteration
and tessellation, without any more redundant initialization. In all
likelihood we won't see the needle move on powerful platforms, but may
help lower end devices.

Change-Id: I457205786766403a760918e779d36ba056d69cde
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/256097
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
7 files changed