QCanvasPainter: Track textures to clean up by id

Store the ids of textures pending removal in a QSet instead of copies
of QCanvasImage in a QList. This avoids the linear search in
removeTextureId() and the O(N*M) list lookups in handleRemoveTextures().

Assisted-by: Claude Code
Pick-to: 6.12
Change-Id: Ib01cf08a00dd2c6e49adbd6983a2eaf5b197ac1b
Reviewed-by: Owais Akhtar <owais.akhtar@qt.io>
Reviewed-by: Kaj Grönholm <kaj.gronholm@qt.io>