Add null pipe
upipe_avcdv: fix documentation

Fix typo preventing doc from building.
Merge branch 'master' of git://github.com/cmassiot/upipe
Update upipe_null with uprobe/ulog refactoring
upipe-gl: Add upipe-gl

GLX sink, unit test, example using avfsrc/avcdv and glx_sink.
This commits does NOT update configure.ac and
{include,lib,tests}/Makefile.am.
Keep GLX-specific and piping code in upipe_glx_sink, and
put gl-only code in an adhoc probe.
Application developers can use their own specific probe to
replace the gl-only code (init, render, reshape).

TODO:
- fix glx-related valgrind errors (non-fatal X leaks/invalid read)
- handle more glx interactions (mouse, move/resize window, ...)
- gl_cocoa_sink
- glut_sink (?)
- text input (OSD ?)
- multiple-input (upipe_input_alloc)
- add a "plain" renderer
- gl-accelerated yuv support (glsl ?)
Update global makefiles/configure for upipe-gl
Fix .pc.in for upipe-gl and upipe-av
Fix includes install paths