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/opengl/Shader.h

@@ -94,6 +94,7 @@ private:
 	MatrixSize getMatrixSize(GLenum type) const;
 	UniformType getUniformBaseType(GLenum type) const;
 	TextureType getUniformTextureType(GLenum type) const;
+	bool isDepthTextureType(GLenum type) const;
 
 	GLuint compileCode(ShaderStage stage, const std::string &code);