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/modules/graphics/Texture.h

@@ -26,9 +26,11 @@
 #include "common/math.h"
 #include "common/pixelformat.h"
 #include "common/Exception.h"
+#include "common/Optional.h"
 #include "Drawable.h"
 #include "Quad.h"
 #include "vertex.h"
+#include "depthstencil.h"
 
 // C
 #include <stddef.h>
@@ -143,6 +145,9 @@ public:
 	virtual bool setMipmapSharpness(float sharpness) = 0;
 	float getMipmapSharpness() const;
 
+	virtual void setDepthSampleMode(Optional<CompareMode> mode = Optional<CompareMode>());
+	Optional<CompareMode> getDepthSampleMode() const;
+
 	Quad *getQuad() const;
 
 	virtual ptrdiff_t getHandle() const = 0;
@@ -185,6 +190,8 @@ protected:
 
 	float mipmapSharpness;
 
+	Optional<CompareMode> depthCompareMode;
+
 	StrongRef<Quad> quad;
 
 private: