Screen: Partially factor out splash image drawing code.

A half-assed move, as the OpenGL-ES code is still in original form.
Prep work for the following OS X hack…
Screen: Allow passing a pixelSize of 30 to Screen('Resolution')

So mode switching to 30 bpp is possible in principle.

-> Screen mex for 64-Bit Octave/Matlab on OS X rebuilt and tested.