Use image data hash as name for temporary pdf files

Closes: #49

_extensions/diagram/diagram.lua

@@ -410,7 +410,10 @@ end
 
 --- Converts a PDF to SVG.
 local pdf2svg = function (imgdata)
-  local pdf_file = os.tmpname() .. '.pdf'
+  -- Using `os.tmpname()` instead of a hash would be slightly cleaner, but the
+  -- function causes problems on Windows (and wasm). See, e.g.,
+  -- https://github.com/pandoc-ext/diagram/issues/49
+  local pdf_file = 'diagram-' .. pandoc.utils.sha1(imgdata) .. '.pdf'
   write_file(pdf_file, imgdata)
   local args = {
     '--export-type=svg',