Add dask backends for rasterize() (#995)

Tile-based parallel rasterization via dask.delayed. New `chunks`
parameter splits the output raster into tiles, rasterizes each
independently, and assembles with da.block.

Polygons are rasterized per-tile (scanline fill is tile-exact).
Lines and points are pre-rasterized at full-raster scale then
distributed to tiles to avoid Bresenham boundary artifacts.

Also adds dask-geopandas input support (.compute() eagerly) and
cuspatial optional import stub.

Author: brendancol

setup.cfg

@@ -56,6 +56,12 @@ optional =
     spatialpandas
     # Optional for gpu_rtx functions. Also requires cupy.
     rtxpy
+dask =
+    dask[array]
+    dask-geopandas
+gpu =
+    cupy
+    cuspatial
 tests =
     flake8
     isort