Code improvements: displayvolume always redraws Volume-Bar

[JOliverasC]

@Edzelf
Firts: it's an awsome project, congratulations and thanks for sharing
I realised that displayvolume is always redrawing the bar and the code should be something similar to (Changes in bold)

void displayvolume()
{
#if defined ( USETFT )
static uint8_t oldvol = 0 ; // Previous volume
uint8_t pos ; // Positon of volume indicator
uint8_t vol = 0 ;

vol = vs1053player.getVolume();
if ( vol != oldvol ) // before ..if ( vs1053player.getVolume() != oldvol )
{
pos = map ( vs1053player.getVolume(), 0, 100, 0, 160 ) ;
tft.fillRect ( 0, 126, pos, 2, RED ) ; // Paint red part
tft.fillRect ( pos, 126, 160 - pos, 2, GREEN ) ; // Paint green part
oldvol = vol;
}
#endif
}