Check for color type, bit depth, `sBIT` when `cICP` indicates 10-bit or 12-bit data

[svgeesus]

ITU H.273, whose values are the data in the cICP chunk, has two enumerations for BT.2020 data. These differ in the number of bits:

• Rec. ITU-R BT.2020-2 (10-bit system)
• Rec. ITU-R BT.2020-2 (12-bit system)

It would be useful to do some additional checks and issue warnings if the image data is not plausibly 10/12 bits per component, respectively:

• Color type needs to be 2 (truecolor) or 6 (Truecolor with alpha), the only two options for RGB data deeper than 8 bits/component
• Bit depth needs to be 16
• sBIT needs to be 10 or 12, respectively.

I feel that all of these should be warnings, not errors.