gdkpixbuf: convert _openslide_gdkpixbuf_read to g_autoptr
gdkpixbuf: convert _openslide_gdkpixbuf_read to g_autoptr

gdkpixbuf_read() has some complex cleanup logic, so leave it alone for
now.

Signed-off-by: Benjamin Gilbert <55d1d0083477f6dd5757d8929dd3b9e09d80bd84@cs.cmu.edu>

openslide/openslide,openslide/openslide,openslide/openslide,openslide/openslide