Make filterMaskGPU use GrTextureProxy

This is a staging CL to position the writePixels in sw_draw_with_mask_filter to be moved to GrSurfaceContext

Change-Id: I808372d30ad4aca4a56125ea75d071f7a3747146
Reviewed-on: https://skia-review.googlesource.com/5926
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed