- minor indentation changes
- fix SELECTARG2 behavior (with help from Lionel Ulmer)
- surface locking/unlocking (only rendering and textures surfaces
  supported now)
- beginning of Target/Front surface support
- try to get D3DTOP_SELECTARG_* working
- implemented D3DTOP_SUBTRACT: currently only if OpenGL1.3 is used, we
  have to use GL_SUBTRACT_ARB for other versions