Enabled AVIF decoding with libheif

[hjmallon]

Description

Requires libheif built with libaom or dav1d/rav1e. Changes to the code just required adding '.avif' to the extensions.

Also updated as much of the docs as I could find to mention AVIF.

Tests

I have added a testsuite file, but it only uses the info command like the other heic file.

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

@hjmallon You were totally correct with the failureok trick that I asked you to remove. I realize now that I was confused, thinking that you were marking the entire test as "ok to fail" (and thus meaningless), but in fact I had forgotten that what that flag actually indicates is that it's ok for any individual command within the test to exit with an error return code, but the test overall will pass or fail based on whether the outputs matched any of the available references.

So sorry to have steered you wrong.

Would you mind restoring what you were originally trying to do with the test, so that the heif test will pass whether or not the avif support is enabled (as long as the output matches either the "heic+avif" or "heic only" reference outputs)?

[hjmallon]

See #2818