thread an SkArenaAlloc into shaders

We'll need something like this to extend the lifetime of our
SkBitmapController::State (and the bitmap and mipmaps it holds)
to match the lifetime of the blitter.  This is particularly
important particularly for kMedium where we're not sampling off
the original image.

Not sure which other effects will need this sort of thing, if any.
I think gradients would just write the colors directly into uniforms.

Change-Id: Idefe620f88113ed436e04a101195b27aec1230db
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/262522
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
10 files changed