video_core: Make global EmuWindow instance part of the base renderer class

Makes the global a member of the RendererBase class. We also change this
to be a reference. Passing any form of null pointer to these functions
is incorrect entirely, especially given the code itself assumes that the
pointer would always be in a valid state.

This also makes it easier to follow the lifecycle of instances being
used, as we explicitly interact the renderer with the rasterizer, rather
than it just operating on a global pointer.

src/yuzu/main.cpp

@@ -402,7 +402,7 @@ bool GMainWindow::LoadROM(const QString& filename) {
 
     system.SetGPUDebugContext(debug_context);
 
-    const Core::System::ResultStatus result{system.Load(render_window, filename.toStdString())};
+    const Core::System::ResultStatus result{system.Load(*render_window, filename.toStdString())};
 
     render_window->DoneCurrent();