Caller-chosen display tile size — render square tiles from non-square BIF

Caller-chosen display tile size — render uniform/square display tiles from
formats that store non-square tiles (legacy BIF).

Added

• Level.StitchedGridFor(tile Size) Size — StitchedGrid for a caller-chosen
  display tile size (ceil(Size/tile)), so a viewer can render uniform/square
  display tiles independent of the stored TileSize.
• Level.StitchedTileInto now uses the dst's dimensions as the display tile
  size on overlapping levels (stitched BIF). The composite is region-based,
  so dst may be any size — a viewer can render e.g. 512×512 display tiles even
  though legacy BIF stores non-square 1024×1360 tiles (which choke renderers
  that assume square). The result is pixel-identical to ReadRegion over
  [tx*dst.W, ty*dst.H, dst.W, dst.H], and the per-source-tile decode-once frame
  cache still applies. Pair it with StitchedGridFor(dstSize). Existing callers
  passing a TileSize-sized dst are unchanged.

Notes

• Scoped to overlapping levels (BIF). Non-overlapping formats store square
  tiles and keep the existing fast path: StitchedTileInto there still behaves
  like DecodedTileInto (dst must equal TileSize); use ReadRegion for an
  arbitrary rectangle. StitchedTile / StitchedGrid (the TileSize-default
  convenience forms) are unchanged.