Restore compatibility with rasterio >= 1.3

[wboykinm]

Fixes #39

Summary

The mbtiler path no longer ran on current rasterio (tested against 1.4.4). Three API breaks fixed:

• rasterio._io.virtual_file_to_buffer was a private API and has been removed. Switched _encode_as_png to the public rasterio.io.MemoryFile.
• The affine profile key was deprecated in rasterio 1.0 in favor of transform. It was being silently ignored, so output PNG tiles had no transform set.
• transform_bounds(..., densify_pts=0) is now rejected when the destination CRS is geographic (densify_pts must be at least 2 if the output is geograpic). Dropped the override; rasterio's default (21) is appropriate here.

Also dropped a brittle assert len(test_bytearray) == 34 in test_webp_writer — the exact webp byte count varies across libwebp / Pillow versions (currently 42 with Pillow 10), and the relative comparison right after it (complex data encodes larger than zero-data) still covers the intent.

Test plan

• pytest — 19/19 pass against rasterio==1.4.4, Pillow==10.x, Python 3.10
• End-to-end CLI smoke against test/fixtures/elev.tif:

  rio rgbify test/fixtures/elev.tif out.mbtiles \
    --base-val -100 --interval 0.1 \
    --min-z 10 --max-z 13 --format webp -j 4
  

  Produces a valid 84 KB mbtiles with tiles at z10–13 (1, 1, 2, 2).