Change API so that CoglPixelBuffer no longer knows its w/h/format

The idea is that CoglPixelBuffer should just be a buffer that can be
used for pixel data and it has no idea about the details of any images
that are stored in it. This is analogous to CoglAttributeBuffer which
itself does not have any information about the attributes. When you
want to use a pixel buffer you should create a CoglBitmap which points
to a region of the attribute buffer and provides the extra needed
information such as the width, height and format. That way it is also
possible to use a single CoglPixelBuffer with multiple bitmaps.

The changes that are made are:

• cogl_pixel_buffer_new_with_size has been removed and in its place is
  cogl_bitmap_new_with_size. This will create a pixel buffer at the
  right size and rowstride for the given width/height/format and
  immediately create a single CoglBitmap to point into it. The old
  function had an out-parameter for the stride of the image but with
  the new API this should be queriable from the bitmap (although there
  is no function for this yet).

• There is now a public cogl_pixel_buffer_new constructor. This takes
  a size in bytes and data pointer similarly to
  cogl_attribute_buffer_new.

• cogl_texture_new_from_buffer has been removed. If you want to create
  a texture from a pixel buffer you should wrap it up in a bitmap
  first. There is already API to create a texture from a bitmap.

This patch also does a bit of header juggling because cogl-context.h
was including cogl-texture.h and cogl-framebuffer.h which were causing
some circular dependencies when cogl-bitmap.h includes cogl-context.h.
These weren't actually needed in cogl-context.h itself but a few other
headers were relying on them being included so this adds the #includes
where necessary.

Reviewed-by: Robert Bragg <robert@linux.intel.com>