bmp: Add support for BMP headers with bitmask
bmp: Add support for BMP headers with bitmask

Header size 52 and 56 failed to load before this patch.

https://bugzilla.gnome.org/show_bug.cgi?id=766890

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf