Crop decompression

[wiredfool]

Fixes #2402 .

Changes proposed in this pull request:

• Add decompression bomb check for image.crop, since it can enlarge images.
• Refactor out checks to _crop, so that we can apply them to any core image object.
• Gif disposal is where the bug in out of memory when processing this GIF #2402 was, it was requesting a 1GP dispose_extents.

Still need to:
- [ ] Generate a gif with extra large extents

• Consider making decompression bombs an error at some level, as now the gif from out of memory when processing this GIF #2402 kicks out a warning, then dies.

[homm]

Consider making decompression bombs an error at some level

+1

[wiredfool]

Since we've done this, there's been a change in the GIF code where the extent changes raise a ValueError, instead of triggering the decompression bomb here. Test is now on the https://github.com/wiredfool/Pillow/tree/dispose_extents_test branch. I've removed it here, rebased on master, and I'm going to merge this pending tests passing.