pynodegl: retain a reference to the capture_buffer in Viewer.configure()
pynodegl: retain a reference to the capture_buffer in Viewer.configure()

Ensures that the capture_buffer object is kept alive during the viewer
lifetime.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl