Drop never documented 'alpha' property from nodedef

Includes minimal support code for practical reasons.
We'll need it for a slightly different purpose next commit.

builtin/game/item.lua

@@ -715,7 +715,6 @@ core.nodedef_default = {
 	--	{name="", backface_culling=true},
 	--	{name="", backface_culling=true},
 	--},
-	alpha = 255,
 	post_effect_color = {a=0, r=0, g=0, b=0},
 	paramtype = "none",
 	paramtype2 = "none",

games/devtest/mods/testnodes/textures.lua

@@ -51,23 +51,16 @@ for a=1,#alphas do
 		groups = { dig_immediate = 3 },
 	})
 
-	-- Transparency set via "alpha" parameter
+	-- Transparency set via texture modifier
 	minetest.register_node("testnodes:alpha_"..alpha, {
 		description = S("Alpha Test Node (@1)", alpha),
-		-- It seems that only the liquid drawtype supports the alpha parameter
-		drawtype = "liquid",
+		drawtype = "glasslike",
 		paramtype = "light",
 		tiles = {
-			"testnodes_alpha.png",
+			"testnodes_alpha.png^[opacity:" .. alpha,
 		},
-		alpha = alpha,
+		use_texture_alpha = true,
 
-
-		liquidtype = "source",
-		liquid_range = 0,
-		liquid_viscosity = 0,
-		liquid_alternative_source = "testnodes:alpha_"..alpha,
-		liquid_alternative_flowing = "testnodes:alpha_"..alpha,
 		groups = { dig_immediate = 3 },
 	})
 end

src/nodedef.cpp

@@ -491,21 +491,6 @@ void ContentFeatures::serialize(std::ostream &os, u16 protocol_version) const
 	writeU8(os, leveled_max);
 }
 
-void ContentFeatures::correctAlpha(TileDef *tiles, int length)
-{
-	// alpha == 0 means that the node is using texture alpha
-	if (alpha == 0 || alpha == 255)
-		return;
-
-	for (int i = 0; i < length; i++) {
-		if (tiles[i].name.empty())
-			continue;
-		std::stringstream s;
-		s << tiles[i].name << "^[noalpha^[opacity:" << ((int)alpha);
-		tiles[i].name = s.str();
-	}
-}
-
 void ContentFeatures::deSerialize(std::istream &is)
 {
 	// version detection
@@ -874,11 +859,6 @@ void ContentFeatures::updateTextures(ITextureSource *tsrc, IShaderSource *shdsrc
 	}
 
 	if (is_liquid) {
-		// Vertex alpha is no longer supported, correct if necessary.
-		correctAlpha(tdef, 6);
-		correctAlpha(tdef_overlay, 6);
-		correctAlpha(tdef_spec, CF_SPECIAL_COUNT);
-
 		if (waving == 3) {
 			material_type = (alpha == 255) ? TILE_MATERIAL_WAVING_LIQUID_OPAQUE :
 				TILE_MATERIAL_WAVING_LIQUID_TRANSPARENT;

src/nodedef.h

@@ -418,15 +418,6 @@ struct ContentFeatures
 	void serialize(std::ostream &os, u16 protocol_version) const;
 	void deSerialize(std::istream &is);
 
-	/*!
-	 * Since vertex alpha is no longer supported, this method
-	 * adds opacity directly to the texture pixels.
-	 *
-	 * \param tiles array of the tile definitions.
-	 * \param length length of tiles
-	 */
-	void correctAlpha(TileDef *tiles, int length);
-
 #ifndef SERVER
 	/*
 	 * Checks if any tile texture has any transparent pixels.

src/script/common/c_content.cpp

@@ -618,7 +618,10 @@ void read_content_features(lua_State *L, ContentFeatures &f, int index)
 	}
 	lua_pop(L, 1);
 
-	f.alpha = getintfield_default(L, index, "alpha", 255);
+	warn_if_field_exists(L, index, "alpha",
+		"Obsolete, only limited compatibility provided");
+	if (getintfield_default(L, index, "alpha", 255) != 255)
+		f.alpha = 0;
 
 	bool usealpha = getboolfield_default(L, index,
 			"use_texture_alpha", false);