For GL+Linux, PlatformGLX will try to open an X11 window
regardless of whether we are doing headless/offscreen rendering
or not.
Here we add an OSMesa platform, which will allow us to avoid
opening any window on Linux. This is particularly useful for
situation where a display is not available, like for CI.
One important detail is that even though we are displaying through
a window, we keep the SDL2 dependency in tact for gltf_viewer.
This is due to the fact that gltf_viewer is built upon
FilamentApp, which is heavily integrated with SDL2. This is mostly
ok since we won't be hitting any path for opening a window due to
gltf_viewer's existing support for headless mode.
