3D: position per-series axes by orientation, not by series index

updateMultiAxis() walked the series list and passed the series index to
updateMultiAxisLabels(), which used it to offset the axis outwards from
the graph background. Every series therefore reserved a slot in every
orientation, whether or not it actually had an axis there.

With four series where only the last one sets axisZ, that single extra Z
axis was drawn at (3 + 1) * 0.3 instead of 0.3, leaving three empty
slots behind it - the layout behaved as if the other three series had a
Z axis too.

Count the positions per orientation instead, advancing only for series
that really do have an axis of that orientation. The graph's own axis
still occupies position 0, so the first per-series axis keeps its (0 +
1) * 0.3 offset.

Pick-to: 6.11 6.12
Fixes: QTBUG-146023
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Change-Id: I73de7ff9210cf8c6250d704578994f8e80681474
Reviewed-by: Sami Varanka <sami.varanka@qt.io>
Reviewed-by: Sakaria Pouke <sakaria.pouke@qt.io>