SGSoundSample constructor changes

- Add an explicit constructor SGSoundSample(const SGPath& file) that
  directly uses the specified path instead of going through
  ResourceManager::findPath().

- Reimplement the existing constructor as
  SGSoundSample(const std::string& file, const SGPath& dir),
  delegating to the new one.

- The first argument of the delegating constructor is now a
  const std::string& instead of a const char*.

Among others, this will allow a better implementation of
FGSoundManager::playAudioSampleCommand(). More explanations can be found
here:

  https://sourceforge.net/p/flightgear/mailman/message/37695786/

simgear/sound/sample.cxx

@@ -79,14 +79,19 @@ std::string SGSoundSampleInfo::random_string()
 // SGSoundSample
 //
 
-// constructor
-SGSoundSample::SGSoundSample(const char *file, const SGPath& currentDir) :
+// Constructor
+SGSoundSample::SGSoundSample(const SGPath& file) :
     _is_file(true)
 {
-    SGPath p = simgear::ResourceManager::instance()->findPath(file, currentDir);
-    _refname = p.utf8Str();
+    _refname = file.utf8Str();
 }
 
+// Delegating constructor that goes through the ResourceManager
+SGSoundSample::SGSoundSample(const string& file, const SGPath& dir) :
+    SGSoundSample(
+        simgear::ResourceManager::instance()->findPath(file, dir))
+{ }
+
 // constructor
 SGSoundSample::SGSoundSample( std::unique_ptr<unsigned char, decltype(free)*>& data,
                               int len, int freq, int format )

simgear/sound/sample.hxx

@@ -30,6 +30,8 @@
 #ifndef _SG_SAMPLE_HXX
 #define _SG_SAMPLE_HXX 1
 
+#include <string>
+
 #include <simgear/math/SGMath.hxx>
 #include <simgear/props/props.hxx>
 #include "soundmgr.hxx"
@@ -259,11 +261,19 @@ public:
     virtual ~SGSoundSample () = default;
 
     /**
-     * Constructor
-     * @param file File name of sound
+     * Constructor that uses the specified path as is
+     * @param file Path to sound file
        Buffer data is freed by the sample group
      */
-    SGSoundSample(const char *file, const SGPath& currentDir);
+    explicit SGSoundSample(const SGPath& file);
+
+    /**
+     * Constructor that goes through ResourceManager::findPath()
+     * @param file File name of sound
+       @param dir “Context” argument for ResourceManager::findPath()
+       Buffer data is freed by the sample group
+     */
+    SGSoundSample(const std::string& file, const SGPath& dir);
 
     /**
      * Constructor.