Enhancement: ensure metadata.background is compatible with color package for single-channel images

[Zirafnik]

Enhancement: ensure metadata.background is compatible with color package for single-channel images

[lovell]

I'm OK with this being a breaking change, as part of a future possible v0.34.0, as it simplifies the API so that the background metadata can always be parsed by the color package and therefore operations that use it such as flatten().

Perhaps a single-channel input could return something like { grey: value } instead? The color package accepts values in the range 0-100 so we would need to scale from libvips' 0-255 range.

[lovell]

The simplest solution here is probably to always return an RGB Object in metadata.background regardless of the number of input channels.

Happy to accept a PR for this, if you're able. We should add a test case for this too as it doesn't appear to be covered by any of the scenarios in test/unit/metadata.js at the moment.

[Zirafnik]

Removing greyscale number return type from .metadata() would be a breaking change for anybody that confidently relies on receiving just the number.

Adding additional parameter type for .flatten() background to include a number, is not a breaking change.