the stack

(To avoid having many empty models in the stack, we reuse the current
model if it's empty. Maybe we shouldn't try to be this clever: not
sure...)

added File->Clear + changed behaviour of File->New (now just adds a
new model)

allow smaller fonts for palettes (test with "-deltafontsize")

use display list for 3d arrows (approx. 2x speedup on my laptop for
large vector views) and spheres

removed static quadrics (wrong when \exists multiple gl contexts),
replaced with correct(?) init in drawContext

- fix bb computation to avoid model jumping around when adding points in z=0 plane
- pass interpol matrices as ref

fix compilation problem with gcc4.3.2