Add a short hand parameter for making a fixed size grid. The existing
xstack layout parameter syntax gets tedious if all one wants is a
matrix like grid of the input streams. Add a grid option to the xstack
filter that simplifies this use case by simply specifying the number of
rows and columns instead of specific x/y co-ordinate for each stream.

Also updating the filter documentation to explain the new option.
Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>