Make luaotfload transparency work with its hooks

Since we added new PDF resource management, transparent font settings
through luaotfload were broken.

It was now possible to fix this thanks to new hooks in luaotfload.
See latex3/luaotfload#212

optex/base/optex.lua

@@ -653,9 +653,11 @@ callback.add_to_callback("pre_shipout_filter", function(list)
     return tonode(list)
 end, "_colors")
 --
--- We also hook into `luaotfload`'s handling of color. Instead of the default
--- behavior (inserting colorstack whatsits) we set our own attribute. The hook
--- has to be registered {\em after} `luaotfload` is loaded.
+-- We also hook into `luaotfload`'s handling of color and transparency. Instead
+-- of the default behavior (inserting colorstack whatsits) we set our own
+-- attribute. On top of that, we take care of transparency resources ourselves.
+--
+-- The hook has to be registered {\em after} `luaotfload` is loaded.
 function optex_hook_into_luaotfload()
     if not luaotfload.set_colorhandler then
         return -- old luaotfload, colored fonts will be broken
@@ -677,10 +679,31 @@ function optex_hook_into_luaotfload()
         setattribute(n, color_attribute, attr)
         return head, n
     end)
+    luatexbase.add_to_callback("luaotfload.parse_transparent", function(input) -- from "00" to "FF"
+        -- in luaotfload: 0 = transparent, 255 = opaque
+        -- in optex:      0 = opaque,      255 = transparent
+        local alpha = tonumber(input, 16)
+        if not alpha then
+            tex.error("Invalid transparency specification passed to font")
+            return nil
+        elseif alpha == 255 then
+            return nil -- this allows luaotfload to skip calling us for opaque style
+        end
+        local transp = 255 - alpha
+        local transpv = fmt("%.3f", alpha / 255)
+        pdf.add_page_resource("ExtGState", fmt("tr%d", transp), pdf_dict{ca = transpv, CA = transpv})
+        pdf.add_page_resource("ExtGState", "tr0", pdf_dict{ca = 1, CA = 1})
+        return transp -- will be passed to the below function
+    end, "optex")
+    luaotfload.set_transparenthandler(function(head, n, transp)
+        setattribute(n, transp_attribute, transp)
+        return head, n
+    end)
 end
 
    -- History:
-   -- 2022-03-07 transparency in the colorize() function, current_tr added 
+   -- 2022-08-24 luaotfload transparency with attributes added
+   -- 2022-03-07 transparency in the colorize() function, current_tr added
    -- 2022-03-05 resources management added
    -- 2021-07-16 support for colors via attributes added
    -- 2020-11-11 optex.lua released