vaapipostproc: make deinterlace-mode behave as expected.

vaapipostproc: make deinterlace-mode behave as expected.

deinterlace-mode didn't behave in the way you'd expect if you have
past experience of the deinterlace element. There were two bugs:

 1. "auto" mode wouldn't deinterlace "interleaved" buffers, only "mixed".
 2. "force" mode wouldn't deinterlace "mixed" buffers flagged as progressive.

Fix these up, and add assertions and error messages to detect cases that
aren't handled.

https://bugzilla.gnome.org/show_bug.cgi?id=726361

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>