local: Fix refcounting and do cleanup in the renderers
local: owr_image_server_add_image_renderer() should take ownership of the renderer
local: Add (transfer none) annotation to the window registry singleton getter
owr: Fix various refcounting issues and memory leaks in the media sources
Add test utilities

Run ./out/<platform>/bin/test-self-view after building to run a simple
audio/video capture and output test.

Run ./out/<platform>/bin/test-send-receive after building to run a test
with one sender and one receiver in the same application.
tests: Update for new API