ska2anm does not work when building librw with gl3 backend

[madebr]

Error is:

Program received signal SIGSEGV, Segmentation fault.
0x000000000042c83d in rw::gl3::openGLFW (openparams=0x0) at /home/maarten/programming/librw/src/gl/gl3device.cpp:1645
1645		glGlobals.winWidth = openparams->width;

Backtrace of running ska2anm without arguments is:

#0  0x000000000042c83d in rw::gl3::openGLFW (openparams=0x0)
    at /home/maarten/programming/librw/src/gl/gl3device.cpp:1645
#1  0x000000000042d3d7 in rw::gl3::deviceSystemGLFW (req=rw::DEVICEOPEN, arg=0x0, n=0)
    at /home/maarten/programming/librw/src/gl/gl3device.cpp:1984
#2  0x000000000040b68c in rw::Engine::open (p=0x0) at /home/maarten/programming/librw/src/engine.cpp:270
#3  0x0000000000404d4d in main (argc=1, argv=0x7fffffffdd98)
    at /home/maarten/programming/librw/tools/ska2anm/ska2anm.cpp:26

[aap]

right we should disable anything but the null build