It will allow to add subpictures blending.

This should fix cursor (un)hiding with some OpenGL back-ends

 It seems that some intel drivers did not like the syntax used to embed the
coefficients directly into the source code.
 Also, it is more flexible and will allow runtime changes.

It is not yet used.

All planar YUV are supported (only YV12, I420, I422, I444 were tested).

For now, the YUV is supposed to follow BT-709 when video height is higher
than 576, and BT-601 otherwise. The YUV range is supposed to be TV, and
the RGB one is supposed to be full range.

In the current state, the behaviour is unchanged on macosx (ie YUY2
is used instead), and when opengl ES is requested (RGB 16 bits).

They are now allocated with picture_New and so they will be suitable for
direct rendering.
The requested number of pictures is now respected when both opengl es and
macosx are not used/defined.

The parameter is not yet used.