Use c_uchar for gl2::TRUE and gl2::FALSE

gl2.rs

@@ -60,8 +60,8 @@ pub static ONE_MINUS_DST_COLOR:      c_uint = 0x0307 as c_uint;
 pub static SRC_ALPHA_SATURATE:       c_uint = 0x0308 as c_uint;
 
 /* Boolean */
-pub static TRUE:                     c_int = 1 as c_int;
-pub static FALSE:                    c_int = 0 as c_int;
+pub static TRUE:                     c_uchar = 1 as c_uchar;
+pub static FALSE:                    c_uchar = 0 as c_uchar;
 
 /* BlendEquationSeparate */
 pub static FUNC_ADD:                 c_uint = 0x8006 as c_uint;