examples/shader: add comments

Updates ebitengine/ebitengine.org#29

examples/shader/lighting.go

@@ -14,14 +14,19 @@
 
 //go:build ignore
 
+// Specify the 'pixel' mode.
 //kage:unit pixels
 
 package main
 
+// Uniform variables.
 var Time float
 var Cursor vec2
 
+// Fragment is the entry point of the fragment shader.
+// Fragment returns the color value for the current position.
 func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
+	// You can define variables with a short variable declaration like Go.
 	pos := dstPos.xy - imageDstOrigin()
 
 	lightpos := vec3(Cursor, 50)
@@ -29,5 +34,8 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
 	normal := normalize(imageSrc1UnsafeAt(srcPos) - 0.5)
 	const ambient = 0.25
 	diffuse := 0.75 * max(0.0, dot(normal.xyz, lightdir))
+
+	// You can treat multiple source images by
+	// imageSrc[N]At or imageSrc[N]UnsafeAt.
 	return imageSrc0UnsafeAt(srcPos) * (ambient + diffuse)
 }