add shaders_write_depth example

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

@@ -0,0 +1,20 @@
+#version 330
+
+// Input vertex attributes (from vertex shader)
+in vec2 fragTexCoord;
+in vec4 fragColor;
+
+// Input uniform values
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+
+// Output fragment color
+out vec4 finalColor;
+
+void main()
+{
+    vec4 texelColor = texture(texture0, fragTexCoord);
+    
+    finalColor = texelColor*colDiffuse*fragColor;
+    gl_FragDepth = 1.0 - finalColor.z;
+}