Fix memory growth when remuxing with add_stream_from_template

start_encoding() was calling avcodec_open2() on codec contexts created
by add_stream_from_template(), fully initializing the codec (e.g.
libx264 allocates x264_t, thread pools, reference frames) even when
the stream is only used for remuxing and never calls encode()/decode().
After freeing, the C heap retains this memory, causing RSS to grow with
each output segment.

Add a _template_initialized flag to CodecContext, set by
add_stream_from_template(). In start_encoding(), contexts with this
flag skip avcodec_open2() — the codec opens lazily via encode() or
decode() if actually needed.

Fixes #2135

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: WyattBlue

av/codec/context.pxd

@@ -14,6 +14,10 @@ cdef class CodecContext:
     # Whether AVCodecContext.extradata should be de-allocated upon destruction.
     cdef bint extradata_set
 
+    # True when created via add_stream_from_template(); start_encoding() skips
+    # avcodec_open2() and lets encode()/decode() open the codec lazily if needed.
+    cdef readonly bint _template_initialized
+
     # Used as a signal that this is within a stream, and also for us to access that
     # stream. This is set "manually" by the stream after constructing this object.
     cdef int stream_index

av/container/output.py

@@ -193,6 +193,7 @@ def add_stream_from_template(
 
         # Construct the user-land stream
         py_codec_context: CodecContext = wrap_codec_context(ctx, codec, None)
+        py_codec_context._template_initialized = True
         py_stream: Stream = wrap_stream(self, stream, py_codec_context)
         self.streams.add_stream(py_stream)
 
@@ -368,12 +369,14 @@ def start_encoding(self):
                 if not ctx.is_open:
                     for k, v in self.options.items():
                         ctx.options.setdefault(k, v)
-                    ctx.open()
 
-                    # Track option consumption.
-                    for k in self.options:
-                        if k not in ctx.options:
-                            used_options.add(k)
+                    if not ctx._template_initialized:
+                        ctx.open()
+
+                        # Track option consumption.
+                        for k in self.options:
+                            if k not in ctx.options:
+                                used_options.add(k)
 
             stream._finalize_for_output()