GLMixer Code

BugFix: replace source reproduce alpha coordinates.

Protecting functions manipulating creation and suppression of sources in
the rendering manager and exposing to undo/redo only user-originated
actions of creation and suppresion (avoid recursive calls).

Implementation of Undo for creation / deletion of sources.

Bug Fix: fill_first_frame when stop using FirstFrameFiller.

Bug Fix: provide framerate for FreeFrame source.

Bug Fix: apply depth when loading FreeFrame source.

Recursive loopback: improved user preference.

Re-implementation of RenderingSource to enable non-recursive loopback.
This required a change in rendering pipeline of sources in Views and
RenderingManager. Added GUI and session file saving for RenderingSource
creation.

Link to GLMixer documentation for OSC.

Implemented preference for saving session on exit. Fixed minor GUI bugs.