Tiff: Support decoding f16 images

[RunDevelopment]

Resolves #2795
Based on #3014

Changes:

• Tiff decoder now decodes Gray/Rgb/Rgba f16 images by converting them to f32.
• Added ExtendedColorType::{L16F, La16F, Rgb16F, Rgba16F}.
• Fixed a bug in testing infra

I added support for decoding f16 TIFF images by converting them to f32. This is a lossless conversion, if a bit inefficient. Importantly, this does not add new dependencies. I used the methods on half::f16 exposed by the tiff crate to avoid adding half to our direct dependencies.

This correctly decodes the f16 test images from image-rs/image-tiff#257. (Interestingly, a lot of programs don't seem to decode f16 correctly and interpret it as u16. From the programs I tested, only GIMP and Photoshop decode them correctly.)

TODO:

• I left a TODO on ExtendedColorType, because I probably messed up the order of variants, which breaks serde backwards compat.
• The f16 -> f32 conversion itself is less efficient than necessary. The half crate has optimized methods for converting a slice of f16s to a slice of f32s, but that requires taking half as a direct dependency. We need to decide whether we want that.