Added Source:getDuration (thanks Boolsheet!)

For streaming sources, the duration may not always be sample-accurate and may return -1 if it cannot be determined at all.

src/modules/sound/wrap_Decoder.h

@@ -34,6 +34,7 @@ Decoder *luax_checkdecoder(lua_State *L, int idx);
 int w_Decoder_getChannels(lua_State *L);
 int w_Decoder_getBitDepth(lua_State *L);
 int w_Decoder_getSampleRate(lua_State *L);
+int w_Decoder_getDuration(lua_State *L);
 extern "C" int luaopen_decoder(lua_State *L);
 
 } // sound