We now set deviceScale to always the mapping from logical window system pixels
to device independent pixels (which is what Ebiten API users expect), and
introduce a new concept of videoModeScale that maps from video mode to logical
window system pixels.
videoModeScale is now only used for computing full-screen resolutions, while
deviceScale is used for any other conversion.
Fixes window sizes on X11, should be a NOP otherwise.
Closes#1774.