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.

doc/reference/cogl/cogl-sections.txt

@@ -488,4 +488,10 @@ CoglBufferAccess
 cogl_buffer_map
 cogl_buffer_unmap
 cogl_buffer_set_data
+
+<SUBSECTION>
+cogl_pixel_buffer_new
+cogl_pixel_buffer_new_for_size
+cogl_is_pixel_buffer
+
 </SECTION>