Fixed bug in audio mixer when one source is muted

git-svn-id: https://gpac.svn.sourceforge.net/svnroot/gpac/trunk/gpac@4274 63c20433-aa62-49bd-875c-5a186b69a8fb
Made audio decoders threaded by default and added -psp option in MP4Box

git-svn-id: https://gpac.svn.sourceforge.net/svnroot/gpac/trunk/gpac@4275 63c20433-aa62-49bd-875c-5a186b69a8fb
making Osmo4 a full screen application

git-svn-id: https://gpac.svn.sourceforge.net/svnroot/gpac/trunk/gpac@4276 63c20433-aa62-49bd-875c-5a186b69a8fb
[bouqueau] robustify '-par' parsing

git-svn-id: https://gpac.svn.sourceforge.net/svnroot/gpac/trunk/gpac@4277 63c20433-aa62-49bd-875c-5a186b69a8fb
Fixed bugs in downloader (crash same file is referenced in different scenes), audio mixer and ttxt concatenation 

git-svn-id: https://gpac.svn.sourceforge.net/svnroot/gpac/trunk/gpac@4278 63c20433-aa62-49bd-875c-5a186b69a8fb
[bouqueau] fix AVC PAR rewrite

git-svn-id: https://gpac.svn.sourceforge.net/svnroot/gpac/trunk/gpac@4279 63c20433-aa62-49bd-875c-5a186b69a8fb
Added SVC with extractor playback, fixed SVC hinting without extractors and misc addons in DASH client

git-svn-id: https://gpac.svn.sourceforge.net/svnroot/gpac/trunk/gpac@4280 63c20433-aa62-49bd-875c-5a186b69a8fb