[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>