Fix black window upon resizing.

I don't know why, but removing the glOrtho call makes the window stop
going black when resizing.

libgag/include/SDLGraphicContext.h

@@ -336,6 +336,7 @@ namespace GAGCore
 		// modifiers
 		virtual bool setRes(int w, int h, Uint32 flags);
 		virtual void setRes(int w, int h) { setRes(w, h, optionFlags); }
+		virtual SDL_Surface *getOrCreateSurface(int w, int h, Uint32 flags);
 		virtual SDL_Rect getRes();
 		virtual bool resChanged();
 		virtual void createGLContext();

libgag/src/GraphicContext.cpp

@@ -2055,11 +2055,25 @@ namespace GAGCore
 		glViewport(0, 0, w, h);
 		glMatrixMode(GL_PROJECTION);
 		glLoadIdentity();
-		glOrtho(0, w, 0, h, -1.0, -1.0);
 		glMatrixMode(GL_MODELVIEW);
 		glLoadIdentity();
 	}
 
+	SDL_Surface* GraphicContext::getOrCreateSurface(int w, int h, Uint32 flags) {
+		if (flags & USEGPU)
+		{
+			if (sdlsurface)
+				SDL_FreeSurface(sdlsurface);
+			// Can't use SDL_GetWindowSurface with OpenGL; the documentation forbids it.
+			sdlsurface = SDL_CreateRGBSurface(0, w, h, 32, 0x00ff0000, 0x0000ff00, 0x000000ff, 0xff000000);
+		}
+		else
+		{
+			sdlsurface = SDL_GetWindowSurface(window);
+		}
+		return sdlsurface;
+	}
+
 	bool GraphicContext::setRes(int w, int h, Uint32 flags)
 	{
 		// check dimension
@@ -2101,27 +2115,31 @@ namespace GAGCore
 		// if window exists, resize it
 		if (window) {
 			SDL_SetWindowSize(window, w, h);
-			sdlsurface = SDL_GetWindowSurface(window);
+			getOrCreateSurface(w, h, flags);
 #ifdef HAVE_OPENGL
 			if (flags & USEGPU)
 			{
 				resetMatrices();
 			}
 #endif
 			setClipRect(0, 0, w, h);
-			nextFrame();
+			//nextFrame();
 		}
 		else {
 			// create the new window and the surface
 			window = SDL_CreateWindow(windowTitle.c_str(), SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, w, h, sdlFlags);
-		}
-		sdlsurface = window != nullptr ? SDL_GetWindowSurface(window) : nullptr;
+			sdlsurface = window != nullptr ? getOrCreateSurface(w, h, flags) : nullptr;
 #ifdef HAVE_OPENGL
-		if (flags & USEGPU)
-		{
-			resetMatrices();
-		}
+			// enable GL context
+			if (flags & USEGPU)
+			{
+				if (!context)
+					createGLContext();
+				resetMatrices();
+			}
 #endif
+		}
+
 		// check surface
 		if (!sdlsurface)
 		{
@@ -2132,11 +2150,6 @@ namespace GAGCore
 		else
 		{
 			_gc = this;
-			// enable GL context
-			if (flags & USEGPU && !context)
-			{
-				createGLContext();
-			}
 			// set _glFormat
 			if ((optionFlags & USEGPU) && (_gc->sdlsurface->format->BitsPerPixel != 32))
 			{