rasterize: honour input order across geometry types (#2064) (#2077)

* rasterize: honour user input order across geometry types (#2064)

merge='first'/'last' decides a pixel's value by the global input
position of the geometry, not by the polygon -> line -> point burn
phase.

Before: [(Point, 9), (Polygon, 1)] with merge='last' returned 9 at
the shared pixel because points always burned after polygons.  Now
it returns 1 because the polygon is the last input.

The fix:

- _classify_geometries returns a per-type int64 global-index array
  alongside the existing props arrays.  GeometryCollection unpacking
  inherits the parent input position.
- A new int64 ``order`` array tracks which input index currently
  owns each pixel.  Per-type burn kernels (scanline, lines, points)
  consult ``should_write(is_first, new_idx, cur_idx)`` before any
  write, so ordered merges gate by global index and commutative
  merges keep their old behaviour.
- Built-in merges dispatch to (merge_fn, should_write) pairs.  User
  callables keep the public (pixel, props, is_first) signature
  paired with the always-write predicate.
- All four backends (numpy, cupy, dask+numpy, dask+cupy) take the
  new plumbing.  Dask tiles slice the per-type global-idx arrays
  with the same boolean masks used for props.

Closes #2064

* rasterize: lock in GC ordering and callable semantics (#2064)

Address review nits with two regression tests:

- GeometryCollection sub-geoms share the parent's input index.
  When two sub-geoms (polygon + point) of the same GC compete for
  a pixel, the gate sees new_idx == cur_idx and skips, so the
  first-burned (polygon) wins.  Documents the policy.

- User callables for merge keep the pre-2064 "last-burned-wins"
  semantics because they pair with the always-write predicate.
  Built-in 'last' returns the polygon (last input); the callable
  returns the point (last burned).

Author: brendancol