Fix scratch caching of GrAttachment textures.
Fix scratch caching of GrAttachment textures.

This changes makes sure we don't try to cache GrAttachments that are
textures since we set their size to be zero.

Bug: b/241822341
Change-Id: Ie4c70b389622f5d0526dfba71a7fc3c4d219b421
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/568616
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>
Commit-Queue: Greg Daniel <39df0a804564ccb6cf75f18db79653821f37c1c5@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia