Fix compare shader(s)

compare_med.glsl

@@ -0,0 +1,20 @@
+// Compute the pixelwise diff between two textures.
+// Computes the diff for each color chanel separately (r, g, b).
+// Assumes both images are not transparent.
+
+// 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-offs)/size;
+	pos.y = 1.0 - pos.y;
+	vec4 color = texture2D(curr, pos);
+	vec4 basecolor = texture2D(base, pos);
+	color.xyz = color.xyz - basecolor.xyz;
+	color.xyz = (vec3(1.0, 1.0, 1.0) + color.xyz)/2.0;
+	gl_FragColor = color;
+}