Add option to create a deferred render target context with mips

We need this since we have texture generators that draw the base level
but nothing more. Thus we want them to be able to directly draw into
a pre allocated mipped target instead of doing a copy later.

TBR: bsalomon@google.com
Bug: skia:
Change-Id: I1dfae0da7153b21b30fdfa51a7061fc255739a1e
Reviewed-on: https://skia-review.googlesource.com/54100
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
18 files changed