compositor: render full screen after startup

There is a history of artifacts after startup. Ccommit 554e5c7a was applied to clear the frame buffer so that "undefined" data would not be used. That "undefined" data often was latest rendered image, and it was actually masking the underlying problem.

On startup, a full paint of the screen is never scheduled for the frame buffer. This causes parts of the render buffer to remain untouched (now black). And when executing a flip, those areas are now black instead of "undefined" or "likely old image".

The problem is clearly visible, when you have background set, and you execute xfwm4 --replace from a terminal. Most of the background will be black instead of the background image. When moving the terminal around it will "fix" the areas it moves over.

Fix this be scheduling a full rendering of the screen once it becomes managed by the compositor.

Signed-off-by: Timo Teräs timo.teras@iki.fi Closes: #684