renderer_opengl: Namespace OpenGL code

Namespaces all OpenGL code under the OpenGL namespace.

Prevents polluting the global namespace and allows clear distinction
between other renderers' code in the future.

src/video_core/video_core.cpp

@@ -10,7 +10,7 @@
 namespace VideoCore {
 
 std::unique_ptr<RendererBase> CreateRenderer(Core::Frontend::EmuWindow& emu_window) {
-    return std::make_unique<RendererOpenGL>(emu_window);
+    return std::make_unique<OpenGL::RendererOpenGL>(emu_window);
 }
 
 } // namespace VideoCore