yuzu, video_core: Screenshot functionality

Allows capturing screenshot at the current internal resolution (native for software renderer), but a setting is available to capture it in other resolutions. The screenshot is saved to a single PNG in the current layout.

src/video_core/video_core.h

@@ -22,4 +22,6 @@ class RendererBase;
  */
 std::unique_ptr<RendererBase> CreateRenderer(Core::Frontend::EmuWindow& emu_window);
 
+u16 GetResolutionScaleFactor(const RendererBase& renderer);
+
 } // namespace VideoCore