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: Add error-checking in blendPixelPtr for invalid BPPs
GRAPHICS: Fix fonts/bdf.cpp to allow for 32bpp to be used
GUI: Allow GUI cursor creation to work with abitrary 2/4Bpp formats.
GUI: Change name of GUI-renderers to remove "16-bit"
GRAPHICS: Allow VectorRenderer and ThemeEngine to init with 4BPP
Merge pull request #359 from rundfunk47/32bitguioverlay

GUI: Add option to render GUI in 32-bits