enabled expressions on x, y, w, h and t parameters for drawgrid and drawbox, added examples
xa: abort if number of channels or sample rate is 0

Fixes #2711.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Merge branch 'drawbox_exprs' of https://github.com/mjmvisser/FFmpeg

* 'drawbox_exprs' of https://github.com/mjmvisser/FFmpeg:
  enabled expressions on x, y, w, h and t parameters for drawgrid and drawbox, added examples

Reviewed-by: Andrey Utkin
Merged-by: Michael Niedermayer <michaelni@gmx.at>