Graphics: Remove superfluous TEXFBOs while reusing existing ones

While the PingPong buffers were always texture-backed, currentScene
and transBuffer used to be backed by renderbuffers, which might have
been more optimized as render targets on older hardware; but since
all buffers in Graphics got switched to being texture backed to allow
blitting via rendering (when hardware blitting isn't available or broken,
eg. on mobile platforms), their reason to exist vanished.

For transBuffer, we can reuse the backbuffer of the PingPong structure,
while currentScene might have been useless from the start.

src/graphics.cpp

@@ -474,9 +474,7 @@ struct GraphicsPrivate
 
 	bool frozen;
 	TEXFBO frozenScene;
-	TEXFBO currentScene;
 	Quad screenQuad;
-	TEXFBO transBuffer;
 
 	/* Global list of all live Disposables
 	 * (disposed on reset) */
@@ -502,26 +500,15 @@ struct GraphicsPrivate
 		TEXFBO::allocEmpty(frozenScene, scRes.x, scRes.y);
 		TEXFBO::linkFBO(frozenScene);
 
-		TEXFBO::init(currentScene);
-		TEXFBO::allocEmpty(currentScene, scRes.x, scRes.y);
-		TEXFBO::linkFBO(currentScene);
-
 		FloatRect screenRect(0, 0, scRes.x, scRes.y);
 		screenQuad.setTexPosRect(screenRect, screenRect);
 
-		TEXFBO::init(transBuffer);
-		TEXFBO::allocEmpty(transBuffer, scRes.x, scRes.y);
-		TEXFBO::linkFBO(transBuffer);
-
 		fpsLimiter.resetFrameAdjust();
 	}
 
 	~GraphicsPrivate()
 	{
 		TEXFBO::fini(frozenScene);
-		TEXFBO::fini(currentScene);
-
-		TEXFBO::fini(transBuffer);
 	}
 
 	void updateScreenResoRatio(RGSSThreadData *rtData)
@@ -721,8 +708,15 @@ void Graphics::transition(int duration,
 
 	setBrightness(255);
 
+	/* The PP frontbuffer will hold the current scene after the
+	 * composition step. Since the backbuffer is unused during
+	 * the transition, we can reuse it as the target buffer for
+	 * the final rendered image. */
+	TEXFBO &currentScene = p->screen.getPP().frontBuffer();
+	TEXFBO &transBuffer  = p->screen.getPP().backBuffer();
+
 	/* Capture new scene */
-	p->compositeToBuffer(p->currentScene);
+	p->screen.composite();
 
 	/* If no transition bitmap is provided,
 	 * we can use a simplified shader */
@@ -735,7 +729,7 @@ void Graphics::transition(int duration,
 		shader.bind();
 		shader.applyViewportProj();
 		shader.setFrozenScene(p->frozenScene.tex);
-		shader.setCurrentScene(p->currentScene.tex);
+		shader.setCurrentScene(currentScene.tex);
 		shader.setTransMap(transMap->getGLTypes().tex);
 		shader.setVague(vague / 256.0f);
 		shader.setTexSize(p->scRes);
@@ -746,7 +740,7 @@ void Graphics::transition(int duration,
 		shader.bind();
 		shader.applyViewportProj();
 		shader.setFrozenScene(p->frozenScene.tex);
-		shader.setCurrentScene(p->currentScene.tex);
+		shader.setCurrentScene(currentScene.tex);
 		shader.setTexSize(p->scRes);
 	}
 
@@ -790,7 +784,7 @@ void Graphics::transition(int duration,
 
 		/* Draw the composed frame to a buffer first
 		 * (we need this because we're skipping PingPong) */
-		FBO::bind(p->transBuffer.fbo);
+		FBO::bind(transBuffer.fbo);
 		FBO::clear();
 		p->screenQuad.draw();
 
@@ -801,7 +795,7 @@ void Graphics::transition(int duration,
 		FBO::clear();
 
 		GLMeta::blitBeginScreen(Vec2i(p->winSize));
-		GLMeta::blitSource(p->transBuffer);
+		GLMeta::blitSource(transBuffer);
 		p->metaBlitBufferFlippedScaled();
 		GLMeta::blitEnd();
 
@@ -945,13 +939,10 @@ void Graphics::resizeScreen(int width, int height)
 	p->screen.setResolution(width, height);
 
 	TEXFBO::allocEmpty(p->frozenScene, width, height);
-	TEXFBO::allocEmpty(p->currentScene, width, height);
 
 	FloatRect screenRect(0, 0, width, height);
 	p->screenQuad.setTexPosRect(screenRect, screenRect);
 
-	TEXFBO::allocEmpty(p->transBuffer, width, height);
-
 	shState->eThread().requestWindowResize(width, height);
 }