some openslide images seem to have null properties

see #1723

see #394

don't rebuild the exif data block if we don't need it

see #394

small tiffsave fixes too

and fix write of XMP write in heic

To avoid potential signed integer overflow (undefined behavior), as
implicit integer promotion means the operand becomes a (signed) int.

Performance: improve LAB to LCH conversion by ~30%

see #1728

Performance: improve XYZ to LAB conversion by ~15%

we were not checking image bounds correctly for very large animated webp
images

thanks pomirleanu

see #1730

Revert "Performance: improve XYZ to LAB conversion by ~15%"

- VIPS_CLIP is faster than fmin/fmax based library calls
- Cast to int to ensure the cubed root LUT is not referenced by NaN

- Remove unnecessary isinf checks as values are clipped
- Remove any ignored out-of-gamut tracking

Performance: safely improve XYZ to LAB conversion by ~12%

Performance: improve scRGB to sRGB conversion by ~8%

Fix undefined-shift in scanline_read_old