Xpra: Ticket #1821: scroll paint error when disabling opengl with python3 client

Scroll paint is enabled since opengl supports it, but when we turn opengl off we revert back to the cairo backing and get this:

2018-04-26 22:06:03,153 error processing draw packet
Traceback (most recent call last):
  File "/usr/lib64/python3.6/site-packages/xpra/client/mixins/window_manager.py", line 1188, in _draw_thread_loop
  File "/usr/lib64/python3.6/site-packages/xpra/client/mixins/window_manager.py", line 1249, in _do_draw
    window.draw_region(x, y, width, height, coding, data, rowstride, packet_sequence, options, [record_decode_time])
  File "/usr/lib64/python3.6/site-packages/xpra/client/client_window_base.py", line 604, in draw_region
    backing.draw_region(x, y, width, height, coding, img_data, rowstride, options, callbacks)
  File "/usr/lib64/python3.6/site-packages/xpra/client/window_backing_base.py", line 590, in draw_region
    self.paint_scroll(img_data, options, callbacks)
  File "/usr/lib64/python3.6/site-packages/xpra/client/window_backing_base.py", line 545, in paint_scroll
    raise NotImplementedError("no paint scroll on %s" % type(self))
NotImplementedError: no paint scroll on <class 'xpra.client.gtk3.cairo_backing.CairoBacking'>
2018-04-26 22:06:05,985 Error drawing on window 1

Thu, 26 Apr 2018 15:42:28 GMT - Antoine Martin: status changed; resolution set

Done in r19085.

Found the magic formulae in this cairo mailing list post from 2005: Scrolling/Copying.

Sat, 23 Jan 2021 05:34:40 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1821