GUI: Fixed offset of shadows
GUI: Fixing dark line present in shadows
GUI: Fixed shadows to look more like mockups
GRAPHICS: Fix VectorRendererSpec for RGBA8888.

Formerly values in the gradient and blending code overflowed and thus caused
incorrect colors. Now there's some special case for 32bpp modes, which needs
slightly more operations but assures a correct output.
GRAPHICS: Fix an assert that prevented rendering fonts in 4bpp
GUI: Added option for 32-bit rendering
SDL: Set BPP according to saved configuration
GUI: Force restart after changing rendering mode
GRAPHICS: Only apply dithering to 16-bit surface
GUI: Allow GUI cursor creation to work with abitrary 2/4Bpp formats.
SDL: Let cursor be rendered in 16bits
GRAPHICS: Change constant of offset to avoid crash in normal render-mode