Expand GrTextureProxy to handle highestFilterMode

Expand GrTextureProxy to handle highestFilterMode

Once TextureProxies aren't instantiated in the TextureSamplers, the they will need to be able to supply this information.

split out of: https://skia-review.googlesource.com/c/10484/ (Omnibus: Push instantiation of GrTextures later (post TextureSampler))
Change-Id: I66555c0746131f565862f7a30d54ff1d458d2062
Reviewed-on: https://skia-review.googlesource.com/15819
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>