mirax: Add support for PNG main image
mirax: Fix mirax-slidedat-bad-image-format test for PNG support
_openslide_init() formatting cleanups
mirax: Support BMP main image via gdk-pixbuf
png: Reject PNGs with alpha or tRNS; we will need to premultiply
mirax: Check for non-JPEG associated images

Even PNG and BMP slides seem to store associated images as JPEGs.
Explicitly check for this.