mesa: Track position/generic0 aliasing in the VAO.

mesa: Track position/generic0 aliasing in the VAO.

Since the first material attribute no longer aliases with
the generic0 attribute, only aliasing between generic0 and
position is left and entirely dependent on the enabled
state of the VAO. So introduce a gl_attribute_map_mode
in the VAO that is used to track how the position
and the generic 0 attribute alias.
Provide a static const array that can be used to
map from vertex program input indices to VERT_ATTRIB_*
indices. The outer dimension of the array is meant to
be indexed directly by the new VAO member variable.
Also provide methods on the VAO to convert bitmasks of
VERT_BIT's from the VAO numbering to the vertex processing
inputs numbering.

v2: s,unsigned char,GLubyte,g
    s,_ATTRIBUTE_MAP_MODE_MAX,ATTRIBUTE_MAP_MODE_MAX,g
    Change comment style, add comments.

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