Add SDL3_mixer.pas

This commit adds a translation of SDL3_mixer. This version corresponds
to upstream commit 4970bacc5954237037b00f8d2797e23b07782b2f.

Author: suve

units/SDL3.pas

@@ -69,6 +69,7 @@ interface
     {$ENDIF}
   {$ENDIF}
 
+{$DEFINE WANT_CWCHAR_T}                   // want wchar_t type in this unit
 {$I ctypes.inc}                           // C data types
 
 { The include file translates