SDL: Change to 32-bit GUI-overlay
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.
SDL: Make initialization of cursor possible with arbitrary bitdepth
SDL: Added function setSurfaceBitDepth, to perform on-the-fly-switching

This function enables on-the-fly switching between bit-depths, so that we
can use one bit-depth while playing a game (in order to not break
engines/scalers) and another one when showing an overlay (so we can use
32-bit colors in the GUI).
SDL: Blit contents of screen onto overlaySurface when overlay is shown

This code changes the behavior of showOverlay, to blit the current
contents of the screen onto the overlay-surface after performing a
conversion to the new bit-depth, so that the content of the screen still
can be on the background of the GMM or debugger, even if the bit-depths
change
GUI: Change name of GUI-renderers to remove "16-bit"
GRAPHICS: Added a feature-mode for 32 bit graphics

This commits adds a feature-mode for 32-bit graphics and enables the
SDL-backend to check against this mode, which can be defined in the
configuration file
SDL: Added some checks before blitting to overlaybackground