Flipmode for OpenScreen() is sometimes a bad idea

[Psychophanta]

A full Screen allows now the flipmode parameter, which is #PB_Screen_WaitSynchronization by default.
This is, there have to be a predetermined way the screen buffer must be flipped.
So then, there is not possible to use a fullscreen in several flipmodes, in other words, now a screen can not be used sometimes with sync, other times without sync, etc. since FlipBuffers() function does not support any parameter.