gtkimcontextime.c: Fix preedit window placement on HiDPI

Hi,

This MR is to fix the placement of the IME preedit window on HiDPI displays on Windows systems when using Chinese or Japanese input. Note that this kind of patch seems to be not necessary for the master/GTK4 branch. From the commit message:

We must also take the scaling factor into account for placing the IME preedit window that is often used for Chinese and Japanese input on Windows.

Please see the attached PNG for what is going on on HiDPI displays-the image on the left is how the preedit window is shown before the patch, and the image on the right is what happens after the patch. Note that the preedit window should not cover the text where the cursor position is, and the preedit window cannot be moved by the user.

With blessings, thank you!