[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>

src/qt_common/CMakeLists.txt

@@ -84,7 +84,7 @@ target_link_libraries(qt_common PRIVATE core Qt6::Core Qt6::Concurrent SimpleIni
 target_link_libraries(qt_common PUBLIC frozen::frozen-headers)
 target_link_libraries(qt_common PRIVATE gamemode::headers frontend_common)
 
-if (NOT APPLE AND ENABLE_OPENGL)
+if (ENABLE_OPENGL)
     target_compile_definitions(qt_common PUBLIC HAS_OPENGL)
 endif()