Rotating a selection around the raw bounding-box centre moved
grid-aligned elements off the grid permanently. sceneBoundingRect()
comes from font metrics and pen widths, so the centre is almost never
a round number: with a pivot of (132.67, 101.11), an element sitting
at x=100 landed at x=133.78, and no further rotation brought it back.
Positions are written with QString::number() (%.6g), which hides the
floating-point noise but keeps the offset, so the diagram ends up
subtly misaligned with no way to repair it from the UI.
Snap the pivot with Diagram::snapToGrid(), which also follows the
user's configured X/Y grid rather than assuming the 10 px default.
Also compute the rotated offset exactly for multiples of 90 degrees
instead of going through qCos()/qSin(). The rotate actions only ever
pass right angles, and qCos(90 deg) is 6.12e-17 rather than 0, so the
trig path added error for no benefit -- four 90 degree steps did not
return a point to where it started. A quadrant is an axis swap, which
is exact; trig is kept as the fallback for any other angle.
With both, four 90 degree rotations of a grid-aligned element return
it exactly to its original position and every intermediate step stays
on the grid.
Reported by plc-user, who hit the same problem rotating graphical
primitives in the Element Editor -- discussion #618.
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