Alist for specifying screen dimensions in millimeters.
The dimensions will be used for `display-mm-height' and
`display-mm-width' if defined for the respective display.
Each element of the alist has the form (display . (width . height)),
e.g. (":0.0" . (287 . 215)).
If `display' equals t, it specifies dimensions for all graphical
displays not explicitly specified.