add Lua function for movie duration

This retrieves the duration from the movie stream.  Thanks to @asarium for pointing me in the right direction.

code/cutscene/Decoder.h

@@ -52,6 +52,7 @@ struct MovieProperties {
 	FrameSize size;
 
 	float fps = -1.0f;
+	float duration = 0.0f;
 
 	FramePixelFormat pixelFormat = FramePixelFormat::Invalid;
 };

code/cutscene/ffmpeg/FFMPEGDecoder.cpp

@@ -503,6 +503,7 @@ MovieProperties FFMPEGDecoder::getProperties() const
 	props.size.height = static_cast<size_t>(m_status->videoCodecPars.height);
 
 	props.fps = static_cast<float>(getFrameRate(m_status->videoStream, m_status->videoCodecCtx));
+	props.duration = static_cast<float>(m_status->videoStream->duration * av_q2d(m_status->videoStream->time_base));
 
 	props.pixelFormat = getPixelFormat(getConversionFormat(m_status->videoCodecPars.pixel_format));
 

code/scripting/api/objs/movie_player.cpp

@@ -53,10 +53,10 @@ ADE_VIRTVAR(FPS, l_MoviePlayer, "number", "Determines the frames per second of t
 {
 	movie_player_h* ph = nullptr;
 	if (!ade_get_args(L, "o", l_MoviePlayer.GetPtr(&ph)))
-		return ade_set_args(L, "i", -1);
+		return ade_set_args(L, "f", -1);
 
 	if (ph == nullptr || !ph->isValid()) {
-		return ade_set_args(L, "i", -1);
+		return ade_set_args(L, "f", -1);
 	}
 
 	if (ADE_SETTING_VAR) {
@@ -66,6 +66,24 @@ ADE_VIRTVAR(FPS, l_MoviePlayer, "number", "Determines the frames per second of t
 	return ade_set_args(L, "f", ph->player()->getMovieProperties().fps);
 }
 
+ADE_VIRTVAR(Duration, l_MoviePlayer, "number", "Determines the duration in seconds of this movie <b>Read-only</b>", "number",
+            "The duration of the movie or -1 if handle is invalid")
+{
+	movie_player_h* ph = nullptr;
+	if (!ade_get_args(L, "o", l_MoviePlayer.GetPtr(&ph)))
+		return ade_set_args(L, "f", -1);
+
+	if (ph == nullptr || !ph->isValid()) {
+		return ade_set_args(L, "f", -1);
+	}
+
+	if (ADE_SETTING_VAR) {
+		LuaError(L, "Tried to modify read-only member!");
+	}
+
+	return ade_set_args(L, "f", ph->player()->getMovieProperties().duration);
+}
+
 ADE_FUNC(update, l_MoviePlayer, "timespan step_time",
          "Updates the current state of the movie and moves the internal timer forward by the specified time span.",
          "boolean", "true if there is more to display, false otherwise")