[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/yuzu/bootmanager.cpp

@@ -1020,8 +1020,7 @@ bool GRenderWindow::InitializeOpenGL() {
 
     return true;
 #else
-    QMessageBox::warning(this, tr("OpenGL not available!"),
-                         tr("Eden has not been compiled with OpenGL support."));
+    QMessageBox::warning(this, tr("OpenGL not available!"), tr("Eden has not been compiled with OpenGL support."));
     return false;
 #endif
 }
@@ -1031,7 +1030,6 @@ bool GRenderWindow::InitializeVulkan() {
     child_widget = child;
     child_widget->windowHandle()->create();
     main_context = std::make_unique<DummyContext>();
-
     return true;
 }