Playing timeline glitches when adding new asset

Submitted by Isaac Smith

Link to original bug (#747417)

Description

Created attachment 301027
Bug example

When adding an asset to a playing GES timeline the video and audio output freeze for 1/10 of a second, annoyingly. The call to add_asset returns immediately, so the hiccup doesn't seem to be coming from my app's main thread. The attached Python code demonstrates the bug.

Attachment 301027, "Bug example":
test.py