v0.60.0 — DZI/SZI Overlap>0 support (+ OverlapMode API, edge-tile ReadRegion fix)

DZI/SZI Overlap > 0 support — clean composited output, new OverlapMode /
TileContentRect API, and a pre-existing bare-DZI/SZI edge-tile ReadRegion fix.

Added

• DZI/SZI Overlap > 0 support. Bare DZI (formats/dzi) and SZI (formats/szi)
  pyramids whose manifest declares a tile overlap now read correctly:
  ReadRegion / ReadRegionScaled / ScaledStrips / StitchedTile return clean,
  overlap-free composited pixels, while raw Tile() / DecodedTile() return the
  on-disk padded tile. New opentile.OverlapMode enum (OverlapNone /
  OverlapBordered / OverlapStitched) on Level; Level.Overlapping is now the
  derived convenience OverlapMode != OverlapNone (value unchanged for every
  previously readable slide). New Level.TileContentRect(col,row) (Region, bool)
  returns the per-tile content crop within a decoded tile. Overlap = 0 reads are
  byte-identical. Validated against libvips overlap_0/overlap_1 DZIs of the same
  slide (region MAD ≈ JPEG noise) plus synthetic lossless PNG fixtures.

Changed

• Level.Overlapping / Level.Grid / Level.TileOverlap field docs reworded to
  distinguish "padded tiles" (OverlapBordered) from "compacted grid"
  (OverlapStitched); the "Grid does not tile Size" property now belongs to
  OverlapMode == OverlapStitched.

Fixed

• Bare DZI/SZI ReadRegion on right/bottom edge tiles. Bare DZI/SZI store edge
  tiles unpadded at their actual clipped size (unlike TIFF's zero-padded edge
  tiles); the region compositor decoded each tile into a fixed TileSize scratch,
  which the strict decoder rejected for a sub-TileSize edge tile
  (dst 256x256 != decoded 176x256), so ReadRegion failed on any region touching
  a right/bottom edge tile. The compositor now decodes unpadded edge tiles into a
  natural-size buffer; byte-identical for padded-tile formats (TIFF/BIF). Pre-existing
  since the v0.52 bare-DZI reader.