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Fix T94137: GPencil: Eraser does not erase first point

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.

Maniphest Tasks: T94137

Differential Revision: https://developer.blender.org/D13593