OPENGL: Properly setup pixel data alignment.

If we do not do this, we might end up with a default alignment of 4, which will
fail (as in the graphics will be messed up) in case the screen resolution is
not divisible by 4.

Thanks to digitall for noticing this problem and finding out about
GL_UNPACK_ALIGNMENT.

backends/graphics/opengl/opengl-graphics.cpp

@@ -341,9 +341,9 @@ void OpenGLGraphicsManager::copyRectToScreen(const byte *buf, int pitch, int x,
 
 	// Copy buffer data to game screen internal buffer
 	const byte *src = buf;
-	byte *dst = (byte *)_screenData.pixels + y * _screenData.pitch;
+	byte *dst = (byte *)_screenData.pixels + y * _screenData.pitch + x * _screenData.bytesPerPixel;
 	for (int i = 0; i < h; i++) {
-		memcpy(dst + x * _screenData.bytesPerPixel, src, w * _screenData.bytesPerPixel);
+		memcpy(dst, src, w * _screenData.bytesPerPixel);
 		src += pitch;
 		dst += _screenData.pitch;
 	}
@@ -1063,6 +1063,8 @@ void OpenGLGraphicsManager::loadTextures() {
 		delete _gameTexture;
 #endif
 
+	uint gameScreenBPP = 0;
+
 	if (!_gameTexture) {
 		byte bpp;
 		GLenum intformat;
@@ -1073,6 +1075,7 @@ void OpenGLGraphicsManager::loadTextures() {
 #else
 		getGLPixelFormat(Graphics::PixelFormat::createFormatCLUT8(), bpp, intformat, format, type);
 #endif
+		gameScreenBPP = bpp;
 		_gameTexture = new GLTexture(bpp, intformat, format, type);
 	} else
 		_gameTexture->refresh();
@@ -1119,6 +1122,14 @@ void OpenGLGraphicsManager::loadTextures() {
 	_overlayNeedsRedraw = true;
 	_cursorNeedsRedraw = true;
 
+	// We need to setup a proper unpack alignment value here, else we will
+	// get problems with the texture updates, in case the surface data is
+	// not properly aligned.
+	// For now we use the gcd of the game screen format and 2, since 2 is
+	// the BPP value for the overlay and the OSD.
+	if (gameScreenBPP)
+		glPixelStorei(GL_UNPACK_ALIGNMENT, Common::gcd<uint>(gameScreenBPP, 2));
+
 #ifdef USE_OSD
 	if (!_osdTexture)
 		_osdTexture = new GLTexture(2, GL_RGBA, GL_RGBA, GL_UNSIGNED_SHORT_5_5_5_1);