cannot subtex :signed-byte use :unsigned-byte
cannot subtex :signed-byte use :unsigned-byte

compile-pixel-format is using #'expand-gl-type-name to make sure the
supported cl-opengl enum names are used. However tex-sub-image-* doesnt
allow :signed-byte as a type, only :unsigned-byte.

Fix takes that into account

djeis97/cepl,bsmr-common-lisp/cepl,cbaggers/cepl