[lib][gfx] fix undefined flush pointer

all places that call flush, check if its null before calling it
but its never actually written to, so it can sometimes point to garbage and just fault

lib/gfx/gfx.c

@@ -614,6 +614,7 @@ gfx_surface *gfx_create_surface(void *ptr, uint width, uint height, uint stride,
         surface->free_on_destroy = true;
     }
     surface->ptr = ptr;
+    surface->flush = NULL;
 
     return surface;
 }