VKEYBD: Make code agnostic of OverlayColor.

This removes the use of OverlayColor in vkeybd and supports both 16 and 32bit
overlays.

This follows the same logic as 1f1d35bd3d31fe3430b9b5227b6127cfd52e52a2. That is that in case of 32bit SDL or OpenGL overlay support we cannot be sure what bit size is actually used on runtime. Thus, the OverlayColor typedef cannot be used anymore.