Convert unsupported colour space to RGB on imagick

[jleckie]

This removes the limitation that only CMYK and RGB are supported on imagemagick. To avoid having to add colourspace support for the dozens of different systems supported by imagemagick, it simply converts other colour spaces to SRGB for simplicity when dealing with colour in imagine.

This will cause problems, if you wanted to handle colour profiles before the conversion, but it seems to work well enough for my use cases.

This has the added benefit of allowing HEIC files to be supported and should resolve #755. HEIC files often are in either dci-p3 (Apple Phones) or CIELAB (Nokia?), or YCbCr even.

In order to get HEIC support I needed to build my own imagemagick and imagick libraries from source. There is a few guides around online for this, obviously you will need to customise for your own environment.
https://gist.github.com/caingougou/e2ebe70057270c208d57ed13ec186b7e

[ausi]

I integrated part of this in my HEIC/AVIF pull request #759.

Do you have any test images that use something different than Imagick::COLORSPACE_YCBCR (that are not RGB or CMYK)?

I think it would be more safe to only add support for YCBCR for now.

[jleckie]

Closing my PR as it has been superseded by @ausi