add can_change_{sample_rate,buffer_size}_while_running() methods to an AudioBackend

Allows the GUI and other stuff to know whether or not changing the SR/bufsize is possible while running, which is about to become useful

libs/ardour/ardour/audio_backend.h

@@ -147,6 +147,17 @@ class AudioBackend {
      */
     virtual uint32_t available_output_channel_count (const std::string& device) const = 0;
 
+    /* Return true if the derived class can change the sample rate of the
+     * device in use while the device is already being used. Return false
+     * otherwise. (example: JACK cannot do this as of September 2013)
+     */
+    virtual bool can_change_sample_rate_when_running () const = 0;
+    /* Return true if the derived class can change the buffer size of the
+     * device in use while the device is already being used. Return false
+     * otherwise. 
+     */
+    virtual bool can_change_buffer_size_when_running () const = 0;
+
     /* Set the hardware parameters.
      * 
      * If called when the current state is stopped or paused,

libs/backends/jack/jack_audiobackend.cc

@@ -935,3 +935,15 @@ JACKAudioBackend::n_physical (unsigned long flags) const
 
 	return c;
 }
+
+bool
+JACKAudioBackend::can_change_sample_rate_when_running () const
+{
+	return false;
+}
+
+bool
+JACKAudioBackend::can_change_buffer_size_when_running () const
+{
+	return true;
+}

libs/backends/jack/jack_audiobackend.h

@@ -61,6 +61,9 @@ class JACKAudioBackend : public AudioBackend {
     uint32_t available_input_channel_count (const std::string& device) const;
     uint32_t available_output_channel_count (const std::string& device) const;
 
+    bool can_change_sample_rate_when_running() const;
+    bool can_change_buffer_size_when_running() const;
+
     int set_device_name (const std::string&);
     int set_sample_rate (float);
     int set_buffer_size (uint32_t);