Document how to clean up graphics resources properly

Change-Id: I9a1477dcc608372120d8d8c9b57d8e15f8e64312
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Make canvas cleanup work propertly...

Depending on which mode we are in, we need to run cleanup
on the right thread and in the right way.

Image canvas doesn't contain any GL resources and can be nuked
right away. The actual QSGTexture is managed by the node, so we
don't need to worry about it.

For FBO it is a bit more complicated.
 - Threaded: We create a cleanup handler that runs makeCurrent
   deleteTexture and doneCurrent and then release the surface
   on the GUI thread.
 - Immediate: Same as threaded, just right away
 - Cooperative: Schedule the texture to be deleted on the
   next sync. The Context doesn't have its own GL context
   in this case, so don't worry about it.

Change-Id: I2d0ae7acfa05561faa52f3cacd767eb18cabaf02
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>