refactor: check camera dimensions against the stream, not the decoder

The mismatch Debug compared the monitor's configured size against the decoder
context's. Both hold the same numbers -- the context is filled from the
stream's codecpar a few lines earlier -- but the stream is the thing actually
describing what the camera sends, and reading it here means the check no longer
needs a decoder to have been opened.

That matters for the next step: this was the camera's last use of the decoder
context for anything other than handing it out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Y6FieTwEXuLhhR4e2yiax

src/zm_ffmpeg_camera.cpp

@@ -653,13 +653,17 @@ int FfmpegCamera::OpenFfmpeg() {
     }
   }  // end if have audio stream
 
+  // Ask the stream, not the decoder. Both carry the same dimensions -- the
+  // decoder context is filled from the stream's codecpar -- but the stream is
+  // what the camera is actually sending, and reading it here means this check
+  // does not depend on a decoder having been opened.
   if (
-    ((unsigned int)mVideoCodecContext->width != width)
+    ((unsigned int)mVideoStream->codecpar->width != width)
     ||
-    ((unsigned int)mVideoCodecContext->height != height)
+    ((unsigned int)mVideoStream->codecpar->height != height)
   ) {
     Debug(1, "Monitor dimensions are %dx%d but camera is sending %dx%d",
-          width, height, mVideoCodecContext->width, mVideoCodecContext->height);
+          width, height, mVideoStream->codecpar->width, mVideoStream->codecpar->height);
   }
 
   // Seed fallback per-frame durations (in each stream's time_base) used to space