rel_to_screen (VECTOR* vector, VIEW* view);
Converts a vector to a screen position, like vec_to_screen.
However the vector is a position relative to the view, and not to the
world - the angle and origin of the view is not considered. Used
for calculating the screen
positions of view entities, like lens
||vector to be converted
||View pointer to be used for the conversion
NULL - Result is outside the view window.
VECTOR* - Result is inside the view window.
if (vec_to_screen(temp,camera) != NULL) // if sun position visible at screen
lens_flare.SHOW = ON;
set(lens_flare,SHOW); // defined entity
temp.z = 200; // temp contains the sun screen position, give 200 quants depth
rel_for_screen(lens_flare.x,camera); // now the lens flare is at sun camera position
screen, vec_for_screen, rel_for_screen