fix offscreen rendering with true offscreen buffer

panda/src/display/graphicsBuffer.cxx

@@ -49,6 +49,7 @@ GraphicsBuffer(GraphicsPipe *pipe, GraphicsStateGuardian *gsg,
   _x_size = x_size;
   _y_size = y_size;
   _has_size = true;
+  _default_display_region->compute_pixels(_x_size, _y_size);
   _open_request = OR_none;
 }