Add Depth compare / shadow sampler support. Officially this is only supported in glsl3 shaders.

- Add Texture:setDepthSampleMode(comparemode). Only works on textures with depth pixel formats. A texture with the depth sample mode set will only work with a depth sampler.
- Add DepthImage, DepthArrayImage, and DepthCubeImage sampler keywords to glsl3.

--HG--
branch : minor

src/common/Optional.h

@@ -23,7 +23,7 @@
 namespace love
 {
 
-// Currently only meant for simple types.
+// Currently only meant for simple and small types.
 template <typename T>
 struct Optional
 {
@@ -40,8 +40,11 @@ struct Optional
 		, value(val)
 	{}
 
-private:
-
+	void set(T val)
+	{
+		hasValue = true;
+		value = val;
+	}
 };
 
 typedef Optional<bool> OptionalBool;