gl-util.h: Add GL ID != operator and TEXFBO::clear

TEXFBO::clear resets the struct into its state right
after creation, nulling all IDs inside so they're invaid.

src/gl-util.h

@@ -42,6 +42,10 @@ struct ID \
 	{  \
 		return gl == o.gl;  \
 	}  \
+	bool operator!=(const ID &o) const \
+	{ \
+		return !(*this == o); \
+	} \
 };
 
 /* 2D Texture */
@@ -236,6 +240,13 @@ struct TEXFBO
 		FBO::del(obj.fbo);
 		TEX::del(obj.tex);
 	}
+
+	static inline void clear(TEXFBO &obj)
+	{
+		obj.tex = TEX::ID(0);
+		obj.fbo = FBO::ID(0);
+		obj.width = obj.height = 0;
+	}
 };
 
 #endif // GLUTIL_H