swscale: add ICC intent enum and option

This setting can be used to infuence the type of tone and gamut mapping used
internally when color space conversions are required. As discussed at VDD'24,
the default was set to relative colorimetric clipping, which is approximately
associative, surjective and idempotent. As such, it roundtrips well, although
it is strictly speaking not associative on out-of-gamut colors.

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2024-03-07
 
 API changes, most recent first:
 
+2024-12-xx - xxxxxxxxxx - lsws 8.13.100 - swscale.h
+  Add enum SwsIntent and SwsContext.intent.
+
 2024-12-15 - xxxxxxxxxx - lavc 61.27.100 packet.h
   Add av_container_fifo_alloc_avpacket().