tnl: Code formatting in t_draw.c

tnl: Code formatting in t_draw.c

So many little changes.  Almost everything is indentation or removal of
trailing whitespace.  There's a couple places where an "else" is moved
to the previous line with the "}".  Some places move a loop variable
declaration to the loop.  One change of assert to unreachable.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4512>