Make the pause screen "transparent"

When paused (ESC or back), you now see the paused game below, darkened,
instead of the usual menu background.

Not enabled in VR mode for now because it could get weird.
Also not trivial in skip-buffered mode (would require taking a screenshot) so
disabled in that case too.

UI/PauseScreen.cpp

@@ -364,6 +364,24 @@ GamePauseScreen::~GamePauseScreen() {
 	__DisplaySetWasPaused();
 }
 
+bool GamePauseScreen::isTransparent() const {
+	// We don't support transparent pause screen in skipbuffereffects mode.
+	return !g_Config.bSkipBufferEffects && !IsVREnabled();
+}
+
+void GamePauseScreen::DrawBackground(UIContext &dc) {
+	if (isTransparent()) {
+		// Darken the game screen coming from below, so the UI on top stands out.
+		dc.Flush();
+		uint32_t color = blackAlpha(0.45f);
+		dc.FillRect(UI::Drawable(color), dc.GetBounds());
+		dc.Flush();
+		return;
+	}
+
+	UIDialogScreenWithGameBackground::DrawBackground(dc);
+}
+
 void GamePauseScreen::CreateViews() {
 	static const int NUM_SAVESLOTS = 5;