[example] Writing into the depth buffer (#2836)

* Add a depth buffer example.

* Fixed a typo

examples/shaders/resources/shaders/glsl330/write_depth.fs

@@ -0,0 +1,13 @@
+#version 330
+
+in vec2 fragTexCoord;
+in vec4 fragColor;
+
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+void main()
+{
+    vec4 texelColor = texture2D(texture0, fragTexCoord);
+    gl_FragColor = texelColor*colDiffuse*fragColor;
+	gl_FragDepth = 1.0 - gl_FragCoord.z;
+}