BeginMenuBar: Fixed minor bug where CursorPosMax gets pushed to CursorPos prior to calling BeginMenuBar(), so e.g. calling the function at the end of a window would often add +ItemSpacing.y to scrolling range.
// We overwrite CursorMaxPos because BeginGroup sets it to CursorPos (essentially the .EmitItem hack in EndMenuBar() would need something analoguous here, maybe a BeginGroupEx() with flags).