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/client/tile.h

@@ -20,6 +20,7 @@ enum MaterialType{
 	TILE_MATERIAL_WAVING_LIQUID_BASIC,
 	TILE_MATERIAL_WAVING_LIQUID_TRANSPARENT,
 	TILE_MATERIAL_WAVING_LIQUID_OPAQUE,
+	// Note: PLAIN isn't a material actually used by tiles, rather just entities.
 	TILE_MATERIAL_PLAIN,
 	TILE_MATERIAL_PLAIN_ALPHA
 };