Remove useless workarounds and use color-mapped custom blits

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 will be a 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 ?