Basic mosaic, video blending modules

Basic mosaic, video blending modules
 * picture.c picture.h : vout that copies the rendered image in a libvlc
                         object.
 * mosaic.c : sub filter that takes all the images created by the picture
              module and blends them over the current video

I think i still have loads of things to fix (like memleaks)

Simple vlm configuration file to see this work :
<<<<
new chan1 broadcast enabled
setup chan1 input somevideo1.avi
setup chan1 option vout=picture

new chan2 broadcast enabled
setup chan2 input somevideo2.avi
setup chan2 option vout=picture

new bg broadcast enabled
setup bg input somevideo0.avi
setup bg option sub-filter=mosaic

control bg play
control chan1 play
control chan2 play
>>>>

You just have to launch : ./vlc -I telnet --vlm-conf vlm.conf