items/mcl_banners: fix preview masking

The `layer` string was inherited from the old patch, which I frankly
don't understand. I restructured it to `follow make_banner_texture` more
closely, with the addition of the mcl_banners_front mask, which
prevents the rear portions of the patterns from showing up.

The numbers in the final `[combine` and `[resize` steps are arbitrary,
and were derived by trial-and-error.

mods/ITEMS/mcl_banners/init.lua

@@ -158,7 +158,7 @@ local make_banner_overlay = function(base_color, layers)
 			local color = mcl_banners.colors[layerinfo.color][4]
 
 			-- Generate layer texture
-			local layer = "(([combine:20x40:-2,-2="..pattern.."^[resize:16x24^[colorize:"..color..":"..layer_ratio..")^[mask:"..pattern..")"
+			local layer = "((("..pattern.."^[mask:mcl_banners_front.png)^[colorize:"..color..":"..layer_ratio..")^[mask:"..pattern..")"
 
 			lovl = (lovl ~= "" and (lovl .. "^") or "") .. layer
 		end
@@ -167,7 +167,7 @@ local make_banner_overlay = function(base_color, layers)
 			 return text:gsub("%^", "\\%^"):gsub(":", "\\:") -- :gsub("%(", "\\%("):gsub("%)", "\\%)")
 		end
 
-		return "[combine:32x32:0,0=" .. escape(base) .. ":8,4=" .. escape(lovl)
+		return "[combine:32x32:0,0=" .. escape(base) .. ":7,3=" .. escape("("..lovl.."^[resize:50x38)")
 	end
 	return base
 end