Add HEIC/HEIF support when libvips version >= 8.8

[vansante]

This PR is largely the work of @jeremysf (see #298)

I have removed all parts that are not HEIF related from that other PR (the version increase, the rotation fix and the scale factor for SVG), in an effort to have a more managable PR and hopefully to get this merged faster :).

Furthermore, I have added an additional mimetype magic bytes sequence for HEIF files, which is different from HEIC files:

HEIC:

pvansanten@paul-thinkpad-tip  ~/Pictures  xxd example.heic | head
00000000: 0000 0018 6674 7970 6865 6963 0000 0000  ....ftypheic....
00000010: 6d69 6631 6865 6963 0000 0f74 6d65 7461  mif1heic...tmeta

HEIF:

pvansanten@paul-thinkpad-tip  ~/Pictures  xxd crowd_1440x960.heic | head 
00000000: 0000 0018 6674 7970 6d69 6631 0000 0000  ....ftypmif1....
00000010: 6d69 6631 6865 6963 0000 01fe 6d65 7461  mif1heic....meta