mesa: Fix and speedup gl_array_object::_MaxElement computation.

mesa: Fix and speedup gl_array_object::_MaxElement computation.

Use a bitmask approach to compute gl_array_object::_MaxElement.
To make this work correctly depending on the shader type actually used,
make use of the newly introduced typed bitmask getters.
With this change I gain about 5% draw time on some osgviewer examples.

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