pngsave: Change default row filter from 'all' to 'none'

[lovell]

As discussed in libvips/devchat.

Preventing the use of adaptive row filtering is usually faster and produces a smaller file size, especially as PNG is typically used for diagrams and line art.

This has been the default behaviour of sharp since early 2018 - see lovell/sharp#872

This will help alleviate #2900

[jcupitt]

I tried a few tests and none is indeed better for line art and synthetic images. It's quite a bit worse for photos, but as you say that's not the usual use of PNG. Let's do it!

[dloebl]

I just wanted to share some data on this change of the default value:

Filter	avg size	p50 size	p99 size	avg processing time	p99 processing time
all	435 KB	137 KB	4800 KB	341 ms	2487 ms
none	480 KB	154 KB	4972 KB	193 ms	1377 ms

Processing times are better. File sizes increased by about 10% on average.
We tested this with 2500 transformation tasks from our PNG traffic (resizes, crops etc).

Libraries in use:
libpng v1.6.37
zlib-ng v2.0.6

[jcupitt]

Yes, I saw about a 2x speedup as well. Thanks for testing!