Add a hook for additional demo work

Useful for beamer.

l3build.dtx

@@ -1146,6 +1146,13 @@
 % apply to a case where multiple \BibTeX{} runs are needed (perhaps where
 % citations can appear within other references).
 %
+% Where there are complex requirements for pre-compiled demonstration
+% files, the hook |typeset_demo_hook()| is available: it runs after
+% copying files to the typesetting location but before the main typesetting
+% run. This may be used for example to script a very large number of
+% demonstrations using a single source (see the \pkg{beamer} package
+% for an example of this).
+%
 % \begin{figure}[!b]
 %   \begin{lstlisting}[frame=single,language={[5.2]Lua},gobble = 6]
 %     #!/usr/bin/env texlua

l3build.lua

@@ -2123,6 +2123,11 @@ function bundlectan()
   return errorlevel
 end
 
+-- A hook to allow additional typesetting of demos
+typeset_demo_tasks = typeset_demo_tasks or function()
+  return 0
+end
+
 -- Typeset all required documents
 -- Uses a set of dedicated auxiliaries that need to be available to others
 function doc(files)
@@ -2145,6 +2150,10 @@ function doc(files)
   unpack({sourcefiles, typesetsourcefiles}, {sourcefiledir, docfiledir})
   -- Main loop for doc creation
   local done = {}
+  local errorlevel = typeset_demo_tasks()
+  if errorlevel ~= 0 then
+    return errorlevel
+  end
   for _, typesetfiles in ipairs({typesetdemofiles, typesetfiles}) do
     for _,i in ipairs(typesetfiles) do
       for _, dir in ipairs({unpackdir, typesetdir}) do