[video_core] Properly disable/avoid building OpenGL when it's disabled (#3692)

- OpenGL symbols would still be included in builds without OpenGL, this pr fixes that
- Same goes for Vulkan, but now with `ENABLE_VULKAN`
- Add support to have OpenGL-only builds (why would you do this?)
- Add support for headless runs (yes you could just select NULL backend, but why not compile it headless? :)

Signed-off-by: lizzie <lizzie@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3692
Reviewed-by: crueter <crueter@eden-emu.dev>
Reviewed-by: DraVee <chimera@dravee.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>
Co-authored-by: lizzie <lizzie@eden-emu.dev>
Co-committed-by: lizzie <lizzie@eden-emu.dev>

0 files changed, 0 insertions, 0 deletions