Fix getTermSize() for Windows

[vnlitvinov]

Original approach used GetSystemMetrics() which actually returned minimal possible size of a window in pixels (most likely even without paying attention to non-zero border sizes).

I'm switching it to already used GetConsoleScreenBufferInfo() (already used in writer_windows.go) which among others returns currently visible window (from which terminal size could be easily calculated).

Without that fix uiprogress on Windows almost always clears the window because it erroneously detects overflows when they aren't actually present.

[cdrage]

I can confirm that this works for me as well, thank you @vnlitvinov

Ping @gosuri