mesa: Factor out struct gl_vertex_format.

mesa: Factor out struct gl_vertex_format.

Factor out struct gl_vertex_format from array attributes.
The data type is supposed to describe the type of a vertex
element. At this current stage the data type is only used
with the VAO, but actually is useful in various other places.
Due to the bitfields being used, special care needs to be
taken for the glGet code paths.

v2: Change unsigned char -> GLubyte.
    Use struct assignment for struct gl_vertex_format.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>