Support images with channel depths > 8 bit

[agoode]

_Imported from Trac. This issue originally by _agoode* at 2010-05-06T16:43:08Z.*

VMU format is 12-bit RGB. Truncating to 8-bit and using the existing read_region API is good enough for now, but we might want to give access to the higher-depth data in some way.

[agoode]

_This comment originally by _agoode* at 2010-06-15T14:11:29Z.*

The correct solution is to add ARGB64 support to cairo and pixman, then to either add a new API call to openslide, or just expose this through openslide_cairo_read_region.

That is a big amount of work though. Try to do it that way, please!

[bgilbert]

pixman 0.46.0 adds support for an a16b16g16r16 format. Nothing has been plumbed into cairo though.

[bgilbert]

In principle we could support this without cairo for formats with tiles at integer positions.