Imagesharp: Add BGP support

[ststeiger]

BPG (Better Portable Graphics) is a new image format.
Its purpose is to replace the JPEG image format when quality or file size is an issue.
Its main advantages are:
High compression ratio. Files are much smaller than JPEG for similar quality.
Supported by most Web browsers with a small Javascript decoder (gzipped size: 56 KB).
Based on a subset of the HEVC open video compression standard.
Supports the same chroma formats as JPEG (grayscale, YCbCr 4:2:0, 4:2:2, 4:4:4) to reduce the losses during the conversion.

An alpha channel is supported.
The RGB, YCgCo and CMYK color spaces are also supported.
Native support of 8 to 14 bits per channel for a higher dynamic range.
Lossless compression is supported.
Various metadata (such as EXIF, ICC profile, XMP) can be included.

Animation support.

https://news.ycombinator.com/item?id=17587684
https://bellard.org/bpg/

[iamcarbon]

I think that ImageSharp needs to stay clear of implementing any patent encumbered formats including BPG and HEIC. This could also be implemented as an external plugin by another developer.