Fix GPU processing mode for Qt 5.1 on Linux.

The GL render context must be created on the thread in which it will be
used. Then, when the consumer stops, we must tell glsl.manager to
cleanup and destroy the context on the render thread.
Add showPaused() method.

This will be used by FiltersDock when it needs to pause the player/MLT
consumer to keep things stable.
Pause consumer instead of restart when change filters list.

Stopping and restarting the consumer is not so stable on Qt 5 with GPU
processing at this time, and it seems heavier handed than simply
pausing. However, to the user an unexpected pause might seem heavier
handed. :-(