Add back tight scaler context managment

Release the scaler context after the differences are calculated so
that the type faces can recoup resources.

Change-Id: Ieb65dafdeee6f84d0739399d58f8d8d53ad7364e
Reviewed-on: https://skia-review.googlesource.com/149233
Reviewed-by: Khusal Sagar <khushalsagar@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>