Fix compare shader(s)

compare.glsl

@@ -4,16 +4,16 @@
 
 // try isampler2D, ivec4 etc.? Probably not
 uniform vec2 size;
+uniform vec2 offs;
 uniform sampler2D base;
 uniform sampler2D curr;
 
 void main(void)
 {
-	vec2 pos = gl_FragCoord.xy/size;
-// 	vec4 color = texture(curr, pos);  // fails for some reason!?
-// 	vec4 basecolor = texture(base, pos);  // fails too...
-// 	color.xyz = abs(color.xyz - basecolor.xyz);
-// 	gl_FragColor = color;
-// 	gl_FragColor = basecolor;
-	gl_FragColor = vec4(1.0);
+	vec2 pos = (gl_FragCoord.xy-offs)/size;
+	pos.y = 1.0 - pos.y;
+	vec4 color = texture2D(curr, pos);
+	vec4 basecolor = texture2D(base, pos);
+	color.xyz = abs(color.xyz - basecolor.xyz);
+	gl_FragColor = color;
 }