Added some more example shaders

shaders/checkerboard_rotate.frag

@@ -0,0 +1,17 @@
+
+// Related examples and context: https://observablehq.com/@mbostock/shader
+
+#define M_PI 3.1415926535897932384626433832795
+
+const float size = 10.0;
+
+mat2 rotate2d(float a) {
+    return mat2(cos(a), -sin(a), sin(a), cos(a));
+}
+
+void mainImage(out vec4 fragColor, in vec2 fragCoord) {
+    float angle = iTime;
+    vec2 p = (fragCoord.xy - iResolution.xy / 2.0) * rotate2d(angle);
+    float k = float(mod(p.x, size * 2.0) < size == mod(p.y, size * 2.0) < size);
+    fragColor = vec4(vec3(k), 1.0);
+}