Improved Animated GIF Support

- Animated GIFS are now loaded differently and use an Octree Quantizer
so now more types of animated gifs are displayed probably.
- Zooming in with the mouse wheel is now zooms in based on the image
center, hold CTRL to zoom based on the top-left like before.
- Fixed problem where images snap back to an old position in some cases.
Limit Zooming

Zooming has been limited to either 75x or 2.5x the screen width (which
ever comes first). This is to prevent the program from crashing at
ridiculous sizes.
Merge branch 'master' of https://github.com/Torrunt/vimage

Conflicts:
	vimage/Source/ImageViewer.cs