scratchpad: fix moving scratchpad window

scratchpad: fix moving scratchpad window

From the source:

    When starting i3 initially (and after each change to the connected
    outputs), this function fixes the resolution of the __i3
    pseudo-output. When that resolution is not set to a function which
    shares a common divisor with every active output’s resolution,
    floating point calculation errors will lead to the scratchpad window
    moving when shown repeatedly.

fixes #632