(SDL tryout - toggling sound/synchronization at runtime + restored silent emulation)
sound.c
- added PAL synchronization during 'silent emulation'
- added feature to start/stop synchronization during emulation (press key 'System Request' or 'Print Screen')
gfxdrsdl.c
- during the flip of the graphical buffer we check for 'silent emulation' and if 20 ms have passed
kbddrvsdl.c
- pressing 'System Request' (also shown as 'Print Screen', 'SysRq' or 'PrtScn') will toggle synchronization to PAL
sounddrvsdl.c
- made changes to support synchronization toggling during emulation (thread stuff)