wrappercamerabinsrc: Add video-source-filter during construction

wrappercamerabinsrc: Add video-source-filter during construction

By adding the video-source-filter during construction time, rather then
patching it in later (*), we can greatly reduce the amount of caps involved
in negotation, speeding up pipeline creation.

I wrote this while working on speeding up the startup of cheese. My cheese
has been modified to add a capsfilter, filtering for only the configured
resolution, with that cheese patch + this patch, the pipeline creation time
goes from aprox 1.1 seconds to aprox 350ms. This is with a Logitech 9000
pro camera, which supports lots of different resolutions at many different
framerates per resolution, causing a caps "explosion" if not filtered.

*) Note the code for this is left in, as it is still necessary if the
video-source-filter is changed between a stop + re-start.

https://bugzilla.gnome.org/show_bug.cgi?id=701953