Handle `ExtraSamples` in GeoTIFF tag

If photometric interpretation is 2 (RGB), and extra samples is non-empty, then it should be rendered as RGB plus the value of extra samples. If the value is 0, that means "unknown"; if 1 or 2 then it means it's an alpha band.

In the case of NAIP data (#184 ) we don't render correctly out of the box because we use the 4th band as alpha. But extra samples is set to 0. This means that the 4th band should be discarded for the purposes of rendering as RGB.

The issue does not name a file or test. Start by locating the GeoTIFF tag handling and the rendering path for RGB data, then trace how ExtraSamples values are interpreted. Done means RGB data with value 0 excludes the extra band, while values 1 or 2 retain it as an alpha band, with NAIP rendering correctly.

Written by the indexing model from the issue text.