LibMedia: Set the time base in FFmpegVideoDecoder

Very minor change, which doesn't actually affect our output, since we
were already inputting and outputting microseconds, but it can't hurt
to give FFmpeg's decoder this information as well.

Author: Zaggy1024

Libraries/LibMedia/FFmpeg/FFmpegVideoDecoder.cpp

@@ -60,7 +60,7 @@ DecoderErrorOr<NonnullOwnPtr<FFmpegVideoDecoder>> FFmpegVideoDecoder::try_create
         return DecoderError::format(DecoderErrorCategory::Memory, "Failed to allocate FFmpeg codec context for codec {}", codec_id);
 
     codec_context->get_format = negotiate_output_format;
-
+    codec_context->time_base = { 1, 1'000'000 };
     codec_context->thread_count = static_cast<int>(min(Core::System::hardware_concurrency(), 4));
 
     if (!codec_initialization_data.is_empty()) {