This allows us to separate accidentally unused, from explicitly unused
parameters, which allows us to turn on such checks in the compiler.

The generally recommended way is to include it from source files, not
headers. We had a mix of both. Let's try to be consistent and follow the
recommended way.

Trying to dynamically track the DPI did not really work as we'd
start accumulating errors and eventually the DPI would start to
drift. Instead maintain a fixed, sensible DPI at all times.

The Xserver (at least Xorg 1.19) rejects RRSetCrtcConfig requests if
rotation is zero.

Change Xserver screen through libXrandr. For complex configurations,
all outputs must have corresponding size modes. As a special case, if
the client requests a single screen with an odd size (for example when
adjusting the size of a non-fullscreen vncviewer), find a smaller
suitable mode, and reduce the framebuffer size as well.