When fractional scaling is used, window sizes from LIBDECOR are still
divided by the window-decorations internal buffer scale which doesn't
match the GHOST window (always 1 in the case of fractional scale).
Also resolve glitches caused by recalculating the window size when
no size change was intended. In this case reuse LIBDECOR's internal
size so there are never any changes (caused by rounding for e.g.).
bafb36f8bc