[graphite] correctly advance index in drawEdgeAAImageSet

Bug: b/513836996
Change-Id: Iff633e0d99443cfc431352cea05051ff567b8f64
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1238179
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
3 files changed