v1.14.0 - codec plugins / svt-av1 backend

• adds SVT-AV1 encoder as AVIF backend
• dynamic plugin interface for installing codecs independently from libheif (currently Unix only)
• add XMP header compression (using zlib)
• library should now be initialized with heif_init()
• heif_convert: extract EXIF and XMP to separate files
• heif_enc: add benchmarking function to compare codecs
• copy EXIF/XMP between HEIC/AVIF files and JPEG/PNG in all kinds of combinations
• remove EXIF orientation tag from input when converting to HEIC/AVIF and generate irot/imir boxes matching the input orientation