The eraser checks the current, previous and next point (and sets pc0,
pc1 & pc2 corresponding to that for futher occlusion/brush/clipping
checks). For the very first point, it sets pc0 to pc1 [which makes sense,
there is no previous point, so we should assume the previous segment is
"visible" as soon as the first point is], but does so before pc1 is
even calculated. This makes following occlusion/brush/clipping checks
work with zero values [which leads to no earsing in most cases].

Now first calculate pc1, then set pc0 to pc1.