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/grayscale.fs

@@ -0,0 +1,20 @@
+#version 330
+
+in vec2 fragTexCoord;
+
+out vec4 fragColor;
+
+uniform sampler2D texture0;
+uniform vec4 tintColor;
+
+// NOTE: Add here your custom variables
+
+void main()
+{
+    vec4 base = texture2D(texture0, fragTexCoord)*tintColor;
+    
+    // Convert to grayscale using NTSC conversion weights
+    float gray = dot(base.rgb, vec3(0.299, 0.587, 0.114));
+    
+    fragColor = vec4(gray, gray, gray, tintColor.a);
+}