diagram-generator: Avoid Inkscape 1.0 deprecation warnings (#227)

Update the arguments to Inkscape, to be in line with new current
Inkscape argument formats (introduced in Inkscape 1.0, released May
2020).

Updating these arguments avoids the following deprecation warnings
(which are safe to ignore, but annoying):

    Warning: Option --without-gui= is deprecated
    Warning: Option --file= is deprecated

and

    Warning: Option --export-plain-svg= is deprecated

if the format is svg, or

    Warning: Option --export-png= is deprecated

if the format is png.

Author: postylem

diagram-generator/diagram-generator.lua

@@ -150,16 +150,27 @@ local tikz_template = [[
 local function convert_with_inkscape(filetype)
   -- Build the basic Inkscape command for the conversion
   local inkscape_output_args
+
+  -- Check inksape version.
+  -- TODO: this can be removed if supporting older Inkscape is not important.
+  local inkscape_v_string = io.popen(inkscape_path .. " --version"):read()
+  local inkscape_v_major = inkscape_v_string:gmatch("([0-9]*)%.")()
+  local isv1 = tonumber(inkscape_v_major) >= 1
+
+  local cmd_arg = isv1 and '"%s" "%s" -o "%s" ' or '"%s" --without-gui --file="%s" '
+
   if filetype == 'png' then
-    inkscape_output_args = '--export-png="%s" --export-dpi=300'
+    local png_arg = isv1 and '--export-type=png' or '--export-png="%s"'
+    output_args = png_arg .. '--export-dpi=300'
   elseif filetype == 'svg' then
-    inkscape_output_args = '--export-plain-svg="%s"'
+    output_args = isv1 and '--export-type=svg --export-plain-svg' or '--export-plain-svg="%s"'
   else
     return nil
   end
+
   return function (pdf_file, outfile)
     local inkscape_command = string.format(
-      '"%s" --without-gui --file="%s" ' .. inkscape_output_args,
+      cmd_arg .. output_args,
       inkscape_path,
       pdf_file,
       outfile