Added an option to build a 16BPP Yabause library.

The new option is YAB_RGB and it let you choose the
pixel format, the two supported values for now are 555 and 565.
Setting this option will disable OpenGL, as I think the 16BPP
mode only makes sense for "full software" renderer, but I may
be wrong.

The Qt port has also been updated to handle the new option,
other ports will display wrong colors. I'm also not 100% sure
it's endian safe...

git-svn-id: https://yabause.svn.sourceforge.net/svnroot/yabause/trunk@3099 d7be59e3-070e-4b9a-85a1-4646befae34d