introduce nodebox abstractions used by fences, like corner nodeboxes, and indirectly slab_x; update fences accordingly

homedecor/handlers/nodeboxes.lua

@@ -4,8 +4,11 @@
 
 -- a box is defined as {x1, y1, z1, x2, y2, z2}
 homedecor.box = {
+	-- slab starting from -x (after rotation: left)
+	slab_x = function(depth) return { -0.5, -0.5, -0.5, -0.5+depth, 0.5, 0.5 } end,
 	-- bottom slab (starting from -y) with height optionally shifted upwards
 	slab_y = function(height, shift) return { -0.5, -0.5+(shift or 0), -0.5, 0.5, -0.5+height+(shift or 0), 0.5 } end,
+	-- slab starting from -z (+z with negative depth)
 	slab_z = function(depth)
 		-- for consistency with the other functions here, we have to assume that a "z" slab starts from -z and extends by depth,
 		-- but since conventionally a lot of nodes place slabs against +z for player convenience, we define
@@ -28,6 +31,14 @@ homedecor.nodebox = {
 	-- { type="regular" },
 	regular = { type="regular" },
 	null = { type = "fixed", fixed = { 0, 0, 0, 0, 0, 0 } },
+	corner_xz = function(depth_x, depth_z) return {
+		type="fixed",
+		fixed={
+			homedecor.box.slab_x(depth_x),
+			homedecor.box.slab_z(depth_z),
+		-- { -0.5, -0.5, -0.5, 0.5-depth, 0.5, -0.5+depth } -- slab_x without the overlap, but actually looks a bit worse
+		}
+	} end,
 }
 
 local mt = {}