add test effects: grayscale and negative

res/shaders/effects/grayscale.glsl

@@ -0,0 +1,5 @@
+vec4 effect() {
+    vec3 color = texture(u_screen, v_uv).rgb;
+    float m = (color.r + color.g + color.b) / 3.0;
+    return vec4(mix(color, vec3(m), u_intensity), 1.0);
+}

res/shaders/effects/negative.glsl

@@ -0,0 +1,6 @@
+vec4 effect() {
+    vec4 color = texture(u_screen, v_uv);
+    color = mix(color, 1.0 - color, u_intensity);
+    color.a = 1.0;
+    return color;
+}