Backing texture does not get updated with the output of a filter call

[codeanticode]

After applying a filter() on a P2D/P3D surface, the contents of the backing texture do not reflect the output of the filter operation. This becomes apparent in this issue reported in the Syphon library: Syphon/Processing#32

The backing texture gets updated in the next frame (probably due to front/back texture swapping), but not clear why the backing texture is not ready by the end of the current frame