(hopefully) proper window style setup in wfreerdp at startup

When connecting with wfreerdp, sometimes the wfreerdp window seems to be not created in full size like here: #2047. That means both scrollbars are shown and the client area of the window seems to be smaller than the requested size (rdp-session resolution). This seems to only happen randomly, but it might be solved by setting up the window's style attributes already with CreateWindowEx(), because when setting the WS_SIZEBOX attribute later (and resizing the window (?)), it seems to end up in unexpected behaviour. The window size is set wrongly, because GetClientRect() seems to return wrong values and canvas diff is set too small? Hopefully this is a real fix, please feel free to correct me anytime!