Fix sub picture region x and y offset values when using --mosaic-keep-picture with a picture that is bigger than on of the mosaic's small rectangles.
Fix sub picture region x and y offset values when using
--mosaic-keep-picture with a picture that is bigger than on of the
mosaic's small rectangles.

This allows creation of mosaics like :
AAAAAABBB
A    ABBB
A    ACCC
AAAAAACCC
(where A, B and C are 3 sub streams)



xkfz007/vlc,xkfz007/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc,jomanmuk/vlc-2.1,krichter722/vlc,krichter722/vlc,jomanmuk/vlc-2.1,shyamalschandra/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,shyamalschandra/vlc,vlc-mirror/vlc,shyamalschandra/vlc,xkfz007/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.2,krichter722/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.1,shyamalschandra/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.2,krichter722/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,krichter722/vlc,jomanmuk/vlc-2.2,xkfz007/vlc