dxx-rebirth: update for fkms and buster #2766
Tested on RPI3/fkms + buster. In GL+SDL2 mode, the D1+2 appears to run at a perfectly smooth 60fps.
My changes to enable mesa support were accepted upstream some time ago, but it doesn't works as expected for buster due to the dependency on GLESv1 functions. Until that's fixed, the SDL2+GL is working very well (and may be more desirable, as SDL2 cannot be enabled for GLES builds).