EGL-vs-GLX should be a run-time decision

[w23]

Currently it is decided at compile time using -DUSE_EGL=1 cmake arg.
One possible approach is to wrap EGL mode into separate target, like libobs-opengl-egl.so.

Things to consider:

• let other parts of the code know about EGL/GLX mode. Xcomp and Xshm captures currently depend on GLX.
• export bare minimum functionality needed for importing GL textures from DMA-BUF fds.