simplify api to bitmapcache

Force all Find callers to make a bitmpacachedesc, which now
has more rigid validation.

Goal is to ensure we never make two desc (which turn into keys)
that look different but represent the same image/transformation.

BUG=skia:

Change-Id: I8571837ee4754a69acc99e949bee9a465fa25f2b
Reviewed-on: https://skia-review.googlesource.com/10114
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
8 files changed