Port yuzu-emu/yuzu#1137: "renderer_opengl: Namespace OpenGL code" (#4423)

* 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.

* Also namespace TextureCubeConfig

src/video_core/renderer_base.cpp

@@ -22,7 +22,7 @@ void RendererBase::RefreshRasterizerSetting() {
         opengl_rasterizer_active = hw_renderer_enabled;
 
         if (hw_renderer_enabled) {
-            rasterizer = std::make_unique<RasterizerOpenGL>(render_window);
+            rasterizer = std::make_unique<OpenGL::RasterizerOpenGL>(render_window);
         } else {
             rasterizer = std::make_unique<VideoCore::SWRasterizer>();
         }