Rio-mbtiles

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There have been no changes since 1.6a2.

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- Turn on latent support for 8-bit input and output.
- Rely on GDAL's warper to create the alpha band for RGB input when --rgba is
used.

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- Add an option to include empty tiles in the output mbtiles dataset. By
default, we continue to exclude them.
- Add an alpha channel for RGB input when the --rgba option is used (76).

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- Add --co (creation) options for fine control over quality of tiles using any
of a format's valid GDAL creation options (73).
- Add support for WebP tiles (72).

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There have been no changes since 1.5b5.

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- Improve estimation of number of tiles.