Fix a problem with the time display

Some changes made in commit e05c827
have broken the update of the time display. This commit fixes the problem
by introducing a second version of MidiTime::ticksToMilliseconds which
takes a double as an argument for the ticks. This new method is then
used by the call to ticksToMilliseconds in Song::processNextBuffer which
fixes the problem.

include/MidiTime.h

@@ -99,6 +99,7 @@ class EXPORT MidiTime
 	static void setTicksPerTact( tick_t tpt );
 	static MidiTime stepPosition( int step );
 	static double ticksToMilliseconds(tick_t ticks, bpm_t beatsPerMinute);
+	static double ticksToMilliseconds(double ticks, bpm_t beatsPerMinute);
 
 private:
 	tick_t m_ticks;

src/core/midi/MidiTime.cpp

@@ -197,6 +197,11 @@ MidiTime MidiTime::stepPosition( int step )
 }
 
 double MidiTime::ticksToMilliseconds(tick_t ticks, bpm_t beatsPerMinute)
+{
+	return MidiTime::ticksToMilliseconds(static_cast<double>(ticks), beatsPerMinute);
+}
+
+double MidiTime::ticksToMilliseconds(double ticks, bpm_t beatsPerMinute)
 {
 	// 60 * 1000 / 48 = 1250
 	return ( ticks * 1250 ) / beatsPerMinute;