Support for swappable tnl modules.

Core Mesa provides a neutral tnl module that verifies the currently
module before installing the tnl function pointers in a lazy fashion.
It also records which tnl functions have been swapped out, and only
restores these when tnl modules themselves are swapped.

Fallback strategies:

Drivers set a bitmask of dangerous stage changes.  When such a state
change occurs, the driver should restore the neutral tnl module via
_mesa_restore_exec_vtxfmt().  The neutral tnl module will call
_mesa_update_state(), followed by ctx->Driver.ValidateTnlModule() if the
validation bitmask matches the new state bitmask.  The driver should
call _tnl_wakeup_exec() if it can no longer handle the current state,
which will revert to the default tnl module.  In this case, previous
vertices should be replayed as required (depending on the current
primitive) after the new tnl module is installed.

If the driver uses chooser functions for any part of the tnl module,
these should generally be reinstalled as part of the fallback to the
neutral tnl module.  For example, if the lighting state changes, a
driver might fall back to the neutral tnl module, verify that the
current lighting state can be handled, and use the chooser function to
pick the most efficient implementation of the current lighting state.

It is up to the drivers to detect and handle fallback cases caused by
tnl function calls themselves (such as glTexCoord4f* if the current tnl
module can't handle projected textures, for example).

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