Fix multithreaded JPEG 32->24 bit conversion

The conversion changed the memory layout, resulting in corruption
on multiprocessor systems.

Keep the same scanline stride across the image, so that the threads
working on different parts of the image don't interfere with each
other.

Also avoid a memcpy, so it should be faster on single-cores too.
Add -q option to be quiet

With -q only errors are sent to stderr, no more diagnostic progress
babble.
Merge branch 'master' of git://git.code.sf.net/p/openpli/aio-grab