gdk-pixbuf 2.36.8 release.

• jpeg: restore grayscale image support (#785171)
• bmp: Tighten image dimension checks (#776694)
• ico: Fixo icon quality sorting (#785447)
• Various other leak and overflow fixes (#765094, #783538, #778204)
• Add some assertion to help static analysis (#778943)
• Remove support for building on various obsolete platforms
• Translation updates