[graphite] Allocate the correct size for text quad vertices

There are 6 vertices per quad, not 1.

Bug: skia:13118
Change-Id: I244f097629511e17306cbeab55abf2db7c170201
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/563457
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
1 file changed