Allow grabing mouse by locking its position

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%402081

src/video/gem/SDL_gemmouse.c

@@ -167,10 +167,12 @@ 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

@@ -113,7 +113,11 @@ GEM_IconifyWindow(_THIS)
 SDL_GrabMode
 GEM_GrabInput(_THIS, SDL_GrabMode mode)
 {
-    return SDL_GRAB_OFF;
+    if (this->screen == NULL) {
+        return SDL_GRAB_OFF;
+    }
+
+    return mode;
 }
 
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