renderer_base: Make Rasterizer() return the rasterizer by reference

All calling code assumes that the rasterizer will be in a valid state,
which is a totally fine assumption. The only way the rasterizer wouldn't
be is if initialization is done incorrectly or fails, which is checked
against in System::Init().

src/video_core/renderer_base.h

@@ -46,8 +46,12 @@ public:
         return m_current_frame;
     }
 
-    RasterizerInterface* Rasterizer() const {
-        return rasterizer.get();
+    RasterizerInterface& Rasterizer() {
+        return *rasterizer;
+    }
+
+    const RasterizerInterface& Rasterizer() const {
+        return *rasterizer;
     }
 
     void RefreshRasterizerSetting();