fix: Check alpha band in Array.as_masked

[kylebarron]

Change list

A GeoTIFF can hold an alpha band. This is one of the bands of the primary data IFD but is defined to hold alpha values that refer to the other bands.

Since the alpha band originates as one of the data bands, we do not remove the alpha band from the primary array.

But in order to match rasterio semantics in as_masked, we need to check for the alpha band and then set mask values according to that band.

• Check for alpha band in as_masked so that we match rasterio semantics.
• Add tests against rasterio

Other references:

• Bilinear resampling creates invalid mask values  rasterio/rasterio#1721 (comment)
• PER_BAND mask and MaskedArray cogeotiff/rio-tiler#580 (comment)
• Current rio-tiler behavior: https://github.com/cogeotiff/rio-tiler/blob/78de6830ce472187510b901fe6f841ddd26a2cfa/rio_tiler/reader.py#L246 and https://github.com/cogeotiff/rio-tiler/blob/ca681fad579e0e480e475a94780e006d0e58c716/rio_tiler/reader.py#L213C12-L213C49
• https://gdal.org/en/stable/development/rfc/rfc15_nodatabitmask.html