Remove useless workarounds and use color-mapped custom blits

The final behavior is exactely the same with and without this patch.

As excepted, the only drawback of the new video system is that the display won't be optimal stretched if resolution is much more than 2 * quest_size.

For exemple on my OSX 10.6, fullscreen modes fill the entiere height and have a little x_offset (because of the 16:10 format, perfectly normal).
But on OSX 10.7, because of the low-resolution abandonment, the display is 640x320 centered on 1024x768 screen, as you said here https://github.com/christopho/solarus/commit/2dfccc74e11c13a4fb4c6971e7879b73dd44ca83

I really think that a feature capable to stretch from any quest_mode size to any resolution size (keeping the same proportionality between each length) is a good idea. Maybe protected by something like a SOLARUS_LINEAR_INTERPOLATION_BLIT flag ?


EDIT: Lelinuxien's report : 
"Le mode large est identique au non large. Sous Windows, j'ai des bords noirs tout autour pour le mode large et le mode plein écran étiré normal n'a aucun bord noir (le jeu occupe tout l'écran).  
Large ou non large, c'est effectivement du 640x480 centré avec du noir tout autour."