h263parse: fix custom picture format (CPFMT) parsing

h263parse: fix custom picture format (CPFMT) parsing

In the H263 spec, CPFMT is present only if the use of a custom
picture format is signalled in PLUSEPTYPE and UFEP is "001",
so we need to check params->format and only if the value is
6 (custom source format) the CPFMT should be read, otherwise
it's not present and wrong data will be parsed.

When reading the CPFMT, the width and height were not
calculated correctly (wrong bitmask).

https://bugzilla.gnome.org//show_bug.cgi?id=749253