Codechange: Implement SoundLoader interface and cache loaded sounds in memory.

Sounds are loaded into memory on first use, using the SoundLoader interface to support format conversion. Sounds are retained in memory to avoid reloading every time a sound is played.

This deduplicates WAV header parsing between NewGRF and baseset sounds, and will allow different audio formats to be supported.

src/soundloader_func.h

@@ -0,0 +1,16 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file soundloader_func.h Functions related to sound loaders. */
+
+#ifndef SOUNDLOADER_FUNC_H
+#define SOUNDLOADER_FUNC_H
+
+bool LoadSound(SoundEntry &sound, SoundID sound_id);
+bool LoadSoundData(SoundEntry &sound, bool new_format, SoundID sound_id, const std::string &name);
+
+#endif /* SOUNDLOADER_FUNC_H */