linux swap interval code breaks offscreen rendering

[Greg Couch]

src/core/graphics/_graphics/linux_swap_interval.cpp links against glXGetProcAddress. When OSMesa is used that generates:

ImportError: /home/gregc/src/chimerax/ChimeraX.app/lib/python3.6/site-packages/chimerax/core/graphics/_graphics.so: undefined symbol: glXGetProcAddress

A workaround would be to use dlsym instead of glXGetProcAddress. Or maybe there is a trick using a weak symbol version of glXGetProcAddress that would work.

[Greg Couch]

Went ahead and made the change to use dlsym. It compiles and doesn't break offscreen rendering, but needs to be tested in GUI mode.

[Tom Goddard]

I took out the dlsym code. Please don't make untested changes that will break VR on linux.

It seems that the problem is glXGetProcAddress is not found because _graphics.so isn't compiled correctly -- it needs to link against the library that contains this symbol. I guess the reason we weren't burned by this compiling mistake in gui mode is because importing PyOpenGL brings in the needed library and is done before importing _graphics.so (except when offscreen is used).

I don't have a linux development system to fix the linking problem, but will fix it when I get time on such a system.

[Tom Goddard]

Fixed.

Linked _graphics.so against libGLX.so on Linux. The vsync control will not be used during offscreen rendering.

[Tom Goddard]

Fixed again.

glXSwapInterval*() is in libGL.so not libGLX.so on Linux.