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.cpp

@@ -230,6 +230,17 @@ float Texture::getMipmapSharpness() const
 	return mipmapSharpness;
 }
 
+void Texture::setDepthSampleMode(Optional<CompareMode> mode)
+{
+	if (mode.hasValue && (!readable || !isPixelFormatDepthStencil(format)))
+		throw love::Exception("Only readable depth textures can have a depth sample compare mode.");
+}
+
+Optional<CompareMode> Texture::getDepthSampleMode() const
+{
+	return depthCompareMode;
+}
+
 Quad *Texture::getQuad() const
 {
 	return quad;