vbo: Use a bitmask to track the active arrays in vbo_exec*.

vbo: Use a bitmask to track the active arrays in vbo_exec*.

The use of a bitmask makes functions iterating only active
attributes less visible in profiles.

v2: Use _mesa_bit_scan{,64} instead of open coding.
v3: Use u_bit_scan{,64} instead of _mesa_bit_scan{,64}.

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