SkPDF: simplify drawImage/Bitmap/Sprite code.

  - SkImageSubset becomes SkKeyedImage
  - SkPDFCanvas::onDraw{Bitmap, Image} go away
  - Remove SkPDFCanvas: base classes now do the right thing.
  - SkPDFDevice::draw{Bitmap,Image}{Rect,}() simplified
  - 244 fewer SLOC.

All but a few PDFs are identical, those rasterize almost the same.

Change-Id: I3ceb3b8935c689719cedf1ad544b0407b5c1733e
Reviewed-on: https://skia-review.googlesource.com/22218
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
10 files changed