HEIF support

[bu6n]

Would you be open to add HEIF support? Nowadays many images are in this format (the most I've seen are coming from Apple devices)

The image library has recently added support for the HEIF magic numbers in https://github.com/elixir-image/image/commit/3face19e86d2e3867804d2c14aa42f021a5a0fd8.

For further info, there's a good documentation here: https://nokiatech.github.io/heif/technical.html

[bu6n]

Here is the code in libheif that parses the size of an image: https://github.com/strukturag/libheif/blob/950936d8425283dac1b95e6461bad40bb621c53c/libheif/context.cc#L172

[rNoz]

Sure, PRs are welcomed :)