added raymarching demo for testing

src/shaders/shapes.cginc

@@ -0,0 +1,14 @@
+float sdSphere(in vec3 pos)
+{
+    return length(pos) - 1.0;
+}
+
+float sdPlane(in vec3 pos)
+{
+    return pos.y;
+}
+
+vec2 sdUnion(vec2 a, vec2 b)
+{
+    return (a.x < b.x) ? a : b;
+}