feat: add Lambertian lighting to fragment shader for surface shading

shaders/simple.frag

@@ -6,5 +6,8 @@ in vec4 vColor;
 in vec3 vNormal;
 
 void main() {
-    color = vec4(vNormal, 1.0);
+    vec3 lightDirection = normalize(vec3(0.8, -0.5, 0.6));
+    float lambert = max(0.0, dot(normalize(vNormal), -lightDirection));
+    vec3 litColor = vColor.rgb * lambert;
+    color = vec4(litColor, vColor.a);
 }