h263parse: update proper PAR calculations

Submitted by Vineeth

Link to original bug (#752690)

Description

In case of h263 video format, libav is calculating the PAR as 12/11
but qtdemux is overwriting it to 1/1, resulting in wrong PAR being passed on.
So instead of just using the PAR from demuxer, check the PAR value in parser,
and compare the demuxer and parser PAR to determine the actual PAR.

This will align the PAR calculation to how it is done in libav, gstavviddec.c