Screen: Change GStreamer videocapture performance optimizations to "off by default"

Performance optimizations during GStreamer videocapture via setting of
“filter-caps” on the video source were on by default, with opt-out via
‘recordingflags’ 4096 on troublesome cameras. Now we change it to an
opt-in: Only apply optimizations if flag 4096 is present instead of
absent.
-> Potential small performance impact, but higher acceptance for weird
cameras.
-> Screen for Matlab on OSX rebuilt and tested.