Allow grabing mouse by locking its position

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%402078

src/video/gem/SDL_gemmouse.c

@@ -176,10 +176,12 @@ void GEM_CheckMouseMode(_THIS)
 
 	/* If the mouse is hidden and input is grabbed, we use relative mode */
 	if ( (!(SDL_cursorstate & CURSOR_VISIBLE)) &&
-		/*(this->input_grab != SDL_GRAB_OFF) && */ /* Damn GEM can not grab */
+		(this->input_grab != SDL_GRAB_OFF) &&
              (SDL_GetAppState() & SDL_APPACTIVE) ) {
+		SDL_AtariXbios_LockMousePosition(SDL_TRUE);
 		GEM_mouse_relative = SDL_TRUE;
 	} else {
+		SDL_AtariXbios_LockMousePosition(SDL_FALSE);
 		GEM_mouse_relative = SDL_FALSE;
 		graf_mouse(M_ON, NULL);
 	}

src/video/gem/SDL_gemwm.c

@@ -108,5 +108,9 @@ int GEM_IconifyWindow(_THIS)
 
 SDL_GrabMode GEM_GrabInput(_THIS, SDL_GrabMode mode)
 {
+	if (this->screen == NULL) {
 		return SDL_GRAB_OFF;
+	}
+
+	return mode;
 }