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.
This should fix fullscreen mode on Linux/X11 systems in general,
while not affecting other systems.
Note that this deletes a bunch of OS X specific and Windows specific code,
as GLFW already provides this functionality.
This change is not expected to cause regressions, however,
the current behavior is still wrong and leads to wrong/unintended window sizes.
To be fixed in further PRs.
Updates #1774
