Fix dispose calculations

- use correct dispose mode
- only apply the dispose on extent of the previous frame
Handle transparency between frames for animated GIFs

Show the previous frame for transparent pixels when the disposal method is 'do not dispose'.

This fixes issue 634.
only update the disposal_method if it not 'unspecified'
ensure that the prevous frame was loaded when seek()ing
Tests dispose and transparency for animated gifs
use assertEqual for test cases
Merge pull request #765 from larsjsol/master

Fix dispose calculations for animated GIFs