Vendor raptor source files (was embedded gitlink)

Tree vendored at upstream 0019c5dd; origin + license recorded in
raptor/VENDOR.txt. Windows SDL dev packages excluded via .gitignore.

raptor/src/dspapi.h

@@ -0,0 +1,14 @@
+#pragma once
+
+struct dsp_t {
+    int16_t format;
+    int16_t freq;
+    int32_t length;
+    char data[1];
+};
+
+void DSP_Init(int channels, int freq);
+void DSP_Mix(int16_t *buffer, int len);
+int DSP_PatchIsPlaying(int handle);
+int DSP_StartPatch(dsp_t* dsp, int sep, int pitch, int volume, int priority);
+void DSP_StopPatch(int handle);