* Add craft-gainmap: ISO 21496-1 and hdrgm gain map metadata, compute and apply, from LightCraft
Moved from lightcraft-codecs' gainmap.rs at 8ef35987 with its tests: the metadata reader and
writer and the arithmetic. The JPEG container (MPF index, two encoded images) stays in the app.
* craft-gainmap: Apple legacy gain-map reconstruction
Apple's legacy single-channel gain maps interpolate a linear gain ratio, not the log2 gain of
ISO 21496-1: apple::apply rebuilds the HDR rendition from a decoded base and the map, with the
headroom taken from the maker-note pair or the HDRGainMapHeadroom tag, checked inputs and
pixel-centre sampling.
Ported from storytold/lightcraft#3760 (which carries #1978).
---------
Co-authored-by: Ted Mao <tedmao@gmail.com>
