SkClipStack: add bounds() and isEmpty() methods

Also: update some of SkPDF to use these rather than the RasterClip.

Change-Id: I5a2564d2269c145135df6a8eda65ab05e4aeaf14
Reviewed-on: https://skia-review.googlesource.com/8159
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
3 files changed