Cleanup wagtailimages backends code
Wand image backend: Always clone images before performing oprations on them
Minor wagtailimages cleanup
Cleanup Filter.process_image a bit
Coalesce animated gifs before resizing them. Fixes #264

In an animated gif, each frame is only the size it needs to be to cover the changed part of the image.
This is often much lower than the actual image size and the Wand image backend was using this incorrect size in its resizing calculations, causing all animated gifs to be resized incorrectly or not at all.

This commit calls ImageMagicks "MagickCoalesceImages" on the wand image. This resizes every frame to the full size of the image so the resizing calculations will work properly.
Revert "Cleanup Filter.process_image a bit"

This reverts commit b5586102ed5ae4bb3f56c33f2dc9de56628fdd2b.
Merge branch 'image-backend-tweaks' of https://github.com/kaedroho/wagtail into kaedroho-image-backend-tweaks
Merge branch 'kaedroho-image-backend-tweaks'