cogl-pixel-buffer: add a pixel buffer object class

cogl-pixel-buffer: add a pixel buffer object class

This subclass of CoglBuffer aims at wrapping PBOs or other system
surfaces like DRM buffer objects. Two constructors are available:

cogl_pixel_buffer_new() with a size when you only care about the size of
the buffer (such a buffer can be used to store several texture data such
as the three planes of a I420 frame).

cogl_pixel_buffer_new_full() is more a 1:1 mapping between the data and
an underlying surface, with the possibility of having access to a low
level memory buffer that may have a stride.