Replace virtual background segmentation model

The original segmentation models used in Jitsi were relicensed by Google
from the Apache license to a proprietary one. While in theory the
previous versions should still be usable under the Apache license it
seems that they were never intended to be released as such. Therefore
the segmentation model is now replaced with the MediaPipe Selfie
Segmentation model.

Code changes are based on commit
"9b6b335c60223dc7615b308b8a25a263c7fc95eb" of repository
"https://github.com/jitsi/jitsi-meet".

"selfie_segmentation_landscape.tflite" was copied from
"mediapipe/modules/selfie_segmentation/selfie_segmentation.tflite" of
repository "https://github.com/google/mediapipe" at commit
"8b57bf879b419173b26277d220b643dac0402334".

"Model Card MediaPipe Selfie Segmentation.pdf" was downloaded from
"https://drive.google.com/file/d/1dCfozqknMa068vVsO2j_1FgZkW_e3VWv/view

".

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>