Convert DstTexture to DstProxy

Convert DstTexture to DstProxy

The last GrTexture-based TextureSampler::reset call must be removed before the TextureSamplers can become purely GrTextureProxy-backed

Split out of: https://skia-review.googlesource.com/c/10484/ (Omnibus: Push instantiation of GrTextures later (post TextureSampler))

Change-Id: Ic1435177d8b5d9bd3fc38b4903c9baae8205cfb0
Reviewed-on: https://skia-review.googlesource.com/16908
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>