Initialize unfilled textures based on GrCaps

This is especially relevant on WebGL, where there
can be a large performance hit to dealing with
uninitialized textures.

Change-Id: Ic8469ec833464a88fe846c72fc02a9af5d43d0d5
Bug: skia:8378
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206396
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>