New: Custom region draw callbacks.
For Martin: an example is now in space_view3d/view3d_edit.c
On middlemouse rotate view, it draws a small square in center.
It works likes this:
#include "ED_space_api.h"
handle= ED_region_draw_cb_activate(region->type, drawfunc, type)
and to stop it:
ED_region_draw_cb_exit(region->type, handle)
drawfunc is of type (const bContext *C, ARegion *ar)
currently it gets called only as type REGION_DRAW_POST, later we
can add more (PRE, POST_XRAY, POST_2D, etc).
For correct usage, these calls should return leaving view transform
unaltered.
2fe5005bbb