This function used to presume a window style used to compute the whole window
size needed for a requested client frame size. The obtained client size of the
window what finally not the one requested - thus the frame buffer was not of
the size obtained by calling GetSize(), leading to problem when capturing
image from the back buffer for instance.

The current window style is now directly obtained using the related Win32 API
function with the window hanle, ensuring a correct window rect computation.