Linux XShm capture plugin

This plugin uses the MIT-SHM extension for the X-server to capture the

desktop.

Todo:

- handle resolution changes of screens

- handle adding/removing screens while recording

- support different depths

Contributing:

If you are interested in helping out with the plugin, please drop by in the

#obs-dev channel on quakenet.

References:

- http://www.x.org/releases/current/doc/xextproto/shm.html