Added some useful postprocessing shaders

Shaders come in two flavours:
- shaders/gl330: OpenGL 3.3+ (Windows, Linux, OSX)
- shaders/gles100:  OpenGL ES 2.0 (Android, RPI, HTML5)

shaders/gl330/posterization.fs

@@ -0,0 +1,26 @@
+#version 330
+
+in vec2 fragTexCoord;
+
+out vec4 fragColor;
+
+uniform sampler2D texture0;
+uniform vec4 tintColor;
+
+// NOTE: Add here your custom variables
+
+float gamma = 0.6f;
+float numColors = 8.0f;
+
+void main()
+{
+    vec3 color = texture2D(texture0, fragTexCoord.xy).rgb;
+    
+    color = pow(color, vec3(gamma, gamma, gamma));
+    color = color*numColors;
+    color = floor(color);
+    color = color/numColors;
+    color = pow(color, vec3(1.0/gamma));
+    
+    fragColor = vec4(color, 1.0);
+}