SDL: Preserve mouse position when switching between OpenGL and Surfac…

…eSDL backends

Since those GraphcisManager initialize the cursor position to (0,0) when
created the cursor was jumping to the top left corner and then moving
back to its initial position as soon as the mouse was moved. Now it
stays at its initial position.

There are still some issues with it when changing between OpenGL and
SurfaceSDL at the same time as toggling fullscreen. But it is not worse
than before.

backends/graphics/sdl/sdl-graphics.cpp

@@ -32,7 +32,9 @@ SdlGraphicsManager::SdlGraphicsManager(SdlEventSource *source, SdlWindow *window
 #if SDL_VERSION_ATLEAST(2, 0, 0)
 	, _allowWindowSizeReset(false), _hintedWidth(0), _hintedHeight(0), _lastFlags(0)
 #endif
-{}
+{
+	SDL_GetMouseState(&_cursorX, &_cursorY);
+}
 
 void SdlGraphicsManager::activateManager() {
 	_eventSource->setGraphicsManager(this);