isl_tab_pip.c: add cuts for all non-integer coordinates in lexmin context

isl_tab_pip.c: add cuts for all non-integer coordinates in lexmin context

The lexmin context applies basically the same algorithm that is
used in the main tableau to the context tableau.
In particular, each cut that is added is immediately used to move
to a lexico-greater point.
It was reported by François Galea, who was working on the implementation
of parametric integer programming in PPL, that first adding cuts
for all non-integer coordinates results in a significant speed
improvement on at least one example.
This patch implements the same idea and results in a similar improvement
on the same example.