Fix the AdjustWindowRectForBorders function so correct window size is computed

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 was 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 frame 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.