Major library cleanups; ops midlayer removed

[Benjamin Gilbert]

Hi all,

I've pushed to master a significant set of cleanups:

- The ops midlayer has been removed. Vendor backends are now responsible 
for both slide parsing and tile rendering.  The reusable parts of the 
ops midlayer have been converted into generic utilities, and the 
format-specific pieces have been moved into the corresponding vendor 
backends.

- The library now retrieves level dimensions, downsample factor, and 
tile size hints from a generic level struct which is instantiated and 
populated by the vendor backend.

- GError is now used for all internal error reporting.

These changes should make the OpenSlide codebase much easier to 
understand and modify.

The introductory wiki page has been updated:

     https://github.com/openslide/openslide/wiki/AddingASlideFormat

--Benjamin Gilbert