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/Text.cpp

@@ -46,7 +46,7 @@ void Text::draw(Graphics *gfx, const Matrix4 &m)
 		return;
 
 	if (Shader::current)
-		Shader::current->checkMainTextureType(TEXTURE_2D);
+		Shader::current->checkMainTextureType(TEXTURE_2D, false);
 
 	gfx->flushStreamDraws();