Swap GrSurfaceProxy over to being held via sk_sp

This CL:
   replaces GrProxyRef with sk_sp
   streamlines GrIORefProxy to be more like SkRefCntBase (i.e., move the fTarget pointer to GrSurfaceProxy)

Change-Id: I17d515100bb2d9104eed64269bd3bf75c1ebbbb8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/221997
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
23 files changed