Updates for OpenSlide 3.4.0

OpenSlide 3.4.0 includes a new API function, `openslide_detect_vendor()`.  Use it when linking against a new-enough OpenSlide.  This solves two problems:

1. For `openslideload` to reject generic TIFF files, it had to do a full `openslide_open()` in order to read the `openslide.vendor` property.  This was slow.

2. If `openslide_open()` failed, we had no way of knowing whether it was an interesting failure (for relevant slide formats) or an uninteresting one (for a generic TIFF).  So, the `is_a` method would always return false in this case.  This could cause unexpected results; for example, on MIRAX slides, VIPS would open the .mrxs file itself (a JPEG thumbnail) and the user would be left wondering where all their pixels went.

Also, some minor fixes and updates.