Use temporary FBO for GrGLGpu::onReadPixels.

Allow configs to be supported as FBO attachments for copies/readbacks without being "renderable" elsewhere in Ganesh.

The motivation for this is to add support for int textures as srcs but not as dsts (at least initially) but to still be able to read them back. This means we don't pay for a unneeded GPU copy when reading back a GrSurface that is a GrTexture and not a GrRenderTarget.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4345

Change-Id: I824a73c6b8c1b9634206d76ccf0848d5f9b64441
Reviewed-on: https://skia-review.googlesource.com/4345
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
5 files changed