Initial support for Sakura .svslide format
sakura: Verify that we have tileids for all tile components
sakura: Read image/tile sizes from header block
sakura: Verify downsamples are positive
sakura: Initialize level metadata earlier
sakura: Allocate tile/channel buffers in caller
Add _openslide_clip_tile() utility function
sakura: Clip right/bottom tiles to level size
jdatasrc: Update from libjpeg-turbo-1.3.0

to get jpeg_mem_src, which isn't available on jpeg-6b/jpeg-7 systems.
sakura: Stop decoding JPEGs via temporary files
sakura: Skip upconversion of channel JPEGs to RGB
jpeg: Rename _openslide_jpeg_read_inner()
sakura: Add associated images
sakura: Drop unnecessary tests

_openslide_jpeg_mem_src() will properly fail on a NULL buffer.
sakura: Add properties

Skip some SVHRScanDataXPO fields for now, since it's not yet clear
whether we're going to see slides containing multiple scans.
hash: Drop unnecessary variable
hash: Add _openslide_hash_data()
sakura: Add quickhash1