QCanvasPainter: Fix possible image cache key collisions

QImage::cacheKey() carries the detach counter in its low 32 bits, so
XORing the image flags there could produce the same key for different
image/flag combinations. Shift the cache key and place the flags in the
low byte instead.

Assisted-by: Claude Code
Pick-to: 6.12
Change-Id: I63a135fa232e7c8be0c81a6a912d3055a3b915ef
Reviewed-by: Owais Akhtar <owais.akhtar@qt.io>
Reviewed-by: Kaj Grönholm <kaj.gronholm@qt.io>