gallium/auxiliary: add contained and rect checks (v6)

gallium/auxiliary: add contained and rect checks (v6)

v3: thanks to Brian, improved coding style, also glennk helped spot few
things (unsigned -> int, two constify)
v4: thanks Ilia improved function, dropped u_box_clip_3d
v5: incorporated rest of Gregor proposed changes,clean ups
v6: u_box_clip_2d simplify proposed by Ilia Mirkin

Acked-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: David Heidelberg <david@ixit.cz>