heifsave: set image orientation using irot and imir transformations

[lovell]

There's previously been some confusion around HEIF and orientation so #1681 made the sensible decision to let libheif deal with this on the input side.

More recently, commit strukturag/libheif@4d9f13b has added support for a similar feature to the output side, which this PR takes advantage of.

Originally reported at lovell/sharp#4289, which includes a sample image.

$ vipsheader --field=orientation in.jpg 
8
$ vips copy in.jpg out.avif

Before:

$ heif-info out.avif 
MIME type: image/avif
main brand: avif
compatible brands: avif, mif1, miaf

image: 810x1080 (id=1), primary
  colorspace: YCbCr, 4:2:0
  bit depth: 8
  color profile: rICC
  alpha channel: no 
  depth channel: no
metadata:
  Exif: 5128 bytes
transformations:
  none
region annotations:
  none

After:

$ heif-info out.avif 
MIME type: image/avif
main brand: avif
compatible brands: avif, mif1, miaf

image: 1080x810 (id=1), primary
  colorspace: YCbCr, 4:2:0
  bit depth: 8
  color profile: rICC
  alpha channel: no 
  depth channel: no
metadata:
  Exif: 5128 bytes
transformations:
  angle (ccw): 90
region annotations:
  none

[kleisauke]

LGTM modulo a small suggestion.

FWIW, this seems to require at least libheif v1.19.0 to function correctly, as earlier versions produce a 810x1080 image with that sample.