Add DVD encode profiles and subtitle hardsub burn-in

Introduce three project-level DVD encode profiles (Compatibility 4500k /
High quality VBR / Custom) via new config keys, and burn a chosen
embedded subtitle track into the picture:

- file_movie carries the hardsub selection (index/kind), persists it, and
  builds a subtitle-picker combobox in its properties page; set_final_rates
  maps the profile to the final video/audio bitrate.
- ffmpeg burns text subs with the subtitles= filter after scaling and image
  subs with an overlay filtergraph before scaling; NTSC DVD audio is now AC3
  for all profiles. A start_offset (-ss) is threaded through for segmenting.
- avconv gets the text-sub path and start_offset for parity.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/devedeng/avconv.py

@@ -112,8 +112,9 @@ class avconv(devedeng.avbase.avbase):
         self.config = devedeng.configuration_data.configuration.get_config()
         self.check_version(["avconv", "-version"])
 
-    def convert_file(self, file_project, output_file, video_length, pass2=False):
+    def convert_file(self, file_project, output_file, video_length, pass2=False, start_offset=0):
 
+        self.start_offset = start_offset
         if file_project.two_pass_encoding:
             if pass2:
                 self.text = _("Converting %(X)s (pass 2)") % {
@@ -123,7 +124,8 @@ class avconv(devedeng.avbase.avbase):
                     "X": file_project.title_name}
                 # Prepare the converting process for the second pass
                 tmp = devedeng.avconv.avconv()
-                tmp.convert_file(file_project, output_file, video_length, True)
+                tmp.convert_file(file_project, output_file, video_length, True,
+                                 start_offset)
                 # it deppends of this process
                 tmp.add_dependency(self)
                 # add it as a child process of this one
@@ -147,6 +149,9 @@ class avconv(devedeng.avbase.avbase):
             self.final_length = video_length
         self.command_var = []
         self.command_var.append("avconv")
+        if start_offset and start_offset > 0:
+            self.command_var.append("-ss")
+            self.command_var.append(str(start_offset))
         self.command_var.append("-i")
         self.command_var.append(file_project.file_name)
 
@@ -159,6 +164,9 @@ class avconv(devedeng.avbase.avbase):
             self.command_var.append("-itsoffset")
             self.command_var.append(str(file_project.audio_delay))
 
+        if start_offset and start_offset > 0:
+            self.command_var.append("-ss")
+            self.command_var.append(str(start_offset))
         self.command_var.append("-i")
         self.command_var.append(file_project.file_name)
         self.command_var.append("-map")
@@ -243,6 +251,16 @@ class avconv(devedeng.avbase.avbase):
                         str(file_project.width_final) + ":h=" + \
                         str(file_project.height_final)
 
+            # text subtitle burn-in (hardsub) as the last filter, after scaling.
+            # Image-based subs need an overlay filtergraph and are only handled
+            # by the ffmpeg backend; avconv users get text subs burned in.
+            if second_pass:
+                text_filter = self._avconv_text_subtitle_filter(file_project)
+                if text_filter is not None:
+                    if (cmd_line != ""):
+                        cmd_line += ",fifo,"
+                    cmd_line += text_filter
+
             if cmd_line != "":
                 self.command_var.append("-vf")
                 self.command_var.append(cmd_line)
@@ -519,6 +537,27 @@ class avconv(devedeng.avbase.avbase):
                 return a
         return 384
 
+    def _avconv_text_subtitle_filter(self, file_project):
+        """ Build a `subtitles=` filter to burn a text-based embedded subtitle
+            track into the picture, or None. Image-based subs are not handled by
+            the avconv backend (use the ffmpeg backend for those). """
+        if getattr(file_project, "hardsub_kind", "text") != "text":
+            return None
+        index = getattr(file_project, "hardsub_index", -1)
+        if index is None or index < 0:
+            return None
+        subs = getattr(file_project, "embedded_subtitles", []) or []
+        rel = None
+        for i, track in enumerate(subs):
+            if track["index"] == index:
+                rel = i
+                break
+        if rel is None:
+            return None
+        path = file_project.file_name.replace("\\", "\\\\").replace(":", "\\:")
+        path = path.replace("'", "\\'")
+        return "subtitles='%s':si=%d" % (path, rel)
+
     def create_menu_mpeg(self, n_page, background_music, sound_length, pal, video_rate, audio_rate, output_path, use_mp2, widescreen):
 
         self.n_page = n_page