Add resolution, like, and merge parameters to rasterize

- resolution: derive width/height from a pixel size (scalar or (dx, dy) tuple)
- like: copy grid dimensions, bounds, and dtype from a template DataArray
- merge: configurable overlap strategy (last/first/max/min/sum/count)
  with per-pixel dispatch via _merge_pixel helpers (CPU and GPU)
- geometry-ID-based scanline ordering so overlap merge follows input order

Author: brendancol