UHDR image support

[ri-char]

This PR adds UHDR image support, leveraging the newly introduced UHDR functionality in libvips (see libvips/libvips#3799). Closed #4314.

New Public Functions

uhdr

uhdr([options]) ⇒ Sharp

Use these UHDR options for output image.

Param	Type	Default	Description
[options]	Object		output options
[options.quality]	number	80	quality, integer 1-100

keepGainmap

keepGainmap() ⇒ Sharp

Keep Gainmap for uhdr image from the input image in the output image. This is the defualt behaviour for uhdr input.

noGainmap

noGainmap() ⇒ Sharp

Remove gainmap from the input image in the output image.

[lovell]

Hello, thank you very much for taking the time to submit a PR - it provides a great start to help people manage images with gain maps.

I intend to take the integration with Ultra HDR much deeper, including it in prebuilt binaries, and supporting gain map scaling/manipulation when resizing/cropping etc. This also means I should probably think more about how this is exposed/implied in the API. I'll create a separate tracking issue with thoughts.