QCanvasPainter: Resolve Start/End alignment for point-based text

fillText() and textBoundingBox() taking x/y/maxWidth built the text
rect from the raw alignment, so the default Start (and End) was
treated as Right. Use the effective, direction-aware alignment like
the rect overloads do.

Assisted-by: Claude Code
Pick-to: 6.12
Change-Id: Ia6d1b51c09ba66265260748cc38c65491d8ac9f7
Reviewed-by: Owais Akhtar <owais.akhtar@qt.io>
Reviewed-by: Kaj Grönholm <kaj.gronholm@qt.io>