FindOpenGL: on macOS, support X11 OpenGL
On macOS, FindOpenGL will not find the OpenGL support for X11 apps, since that uses different library names than the macOS OpenGL frameworks. This MR adds the appropriate names and a bit of explanatory commentary.
Discussion at https://discourse.cmake.org/t/x11-and-opengl-builds-on-macos/9466
Topic-rename: FindOpenGL-macOS-x11
Edited by Brad King