Initial refactoring on shader usage and generation

`IShaderSource` was designed with the idea that if you want a shader,
you must want it for a node. So it depends heavily on being given a tile
material and the node drawtype. But this doesn't make sense neither in theory
nor in practice.
This commit takes a small step towards removing the incorrect abstraction.

src/nodedef.h

@@ -222,6 +222,7 @@ enum NodeDrawType : u8
 	NDT_MESH,
 	// Combined plantlike-on-solid
 	NDT_PLANTLIKE_ROOTED,
+
 	// Dummy for validity check
 	NodeDrawType_END
 };