[387/906] glmixer: add a glmixer base element

[387/906] glmixer: add a glmixer base element

glmixer can be seen as a glfilter except it handles N requested
sink pads.
Each sink pad and the src pad are video/x-raw-gl.
glmixer is responsible for managing different framerates from inputs.
It uses OpenGL context sharing. It means that each input is in its
own OpenGL context shared together and shared with the OpenGL context
of the ouput gl chain.
Also add a glmosaic which is an example of implementation of glmixer.
For now glmosaic is a cube but it will be fixed in the next commits.
For now the glmixer has some weird behaviours in some configurations
but it will be improved in the next commits.
The autotools builds is temporarly broken since those changes
have been made on win32.