SkScalerContext::getImage less brittle.

Properly handle edge cases like
 * the temporary glyph being a different size than expected
 * filters which reduce in size
 * filters which return false to indicate no filtering has been done

Bug: chromium:1190525
Change-Id: Ibc53eb1d7014210019e96cd6bae3e256d967be54
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/392156
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Herb Derby <herb@google.com>
(cherry picked from commit 348ee387a96d7d94733d46ad9e82b19cb890dd16)
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/392437