Allow images to be unloaded to disk or compressed in memory. Provide thumbnails in all tree windows (hotlist, history). Optimise the application initialisation times. Part 2 of 2.
svn path=/import/netsurf/; revision=1762
Thumnails now created with a palette to fix incorrect colour rendition.
Thumnails output to a 32bpp sprite where possible.
Thumnails oversampled to improve display if desired.
svn path=/import/netsurf/; revision=744
