Consolodating description of source extraction in CHANGES.txt.

Extracted do_floodfill, playsound, draw_progressbar and RGB-to-linear functions.

src/playsound.h

@@ -0,0 +1,13 @@
+#ifndef PLAYSOUND_H
+#define PLAYSOUND_H
+
+#include "SDL_mixer.h"
+#include "sounds.h"
+
+extern Mix_Chunk * sounds[NUM_SOUNDS];
+extern int mute, use_sound;
+
+void playsound(int chan, int s, int override);
+
+#endif
+