Custom frontends that enable OpenGL redirection and report support only for H3DOR_FMT_RGBA receive improper callbacks.

[rbundulis]

While implementing 3D redirection in my custom framebuffer I think I've discovered a bug - the H3DOR_FMT_RGBA format is not handled properly. If you look at display_vrdp.cpp:345 then if the returned strings do not match H3DOR_FMT_RGBA_TOPDOWN then successful return code is returned but the initialization functions are not called which results in my framebuffer getting a null pointer in all the instance parameters passed to the calls. I guess since the vrdp server sets both everything goes fine and only issues can be experienced with external framebuffers so this is a minor issue.