Send approximate physical screen dimensions to the VM

When properly set, applications will have a chance to automatically
detect HiDPI and act accordingly. This is the case for Fedora 23
template and GNOME apps (maybe even all built on top of GTK).

But for privacy reasons, don't provide real values, only some
approximate one. Give enough information to distinguish DPI above 150,
200 and 300. This is some compromise between privacy and HiDPI support.

QubesOS/qubes-issues#1951

screen-layout-handler/monitorlayoutnotify.py

@@ -40,6 +40,11 @@
            (?P<x>\d+)[+]
            (?P<y>\d+)
          )|[\D])                       # or not a digit
+        ([ ]\(.*\))?[ ]?               # ignore options
+        (                              #  304mm x 228mm
+           (?P<width_mm>\d+)mm[ ]x[ ]
+           (?P<height_mm>\d+)mm
+        )?
         .*                             # ignore rest of line
         """)
 
@@ -50,11 +55,34 @@ def get_monitor_layout():
         if not line.startswith("Screen") and not line.startswith(" "):
             output_params = REGEX_OUTPUT.match(line).groupdict()
             if output_params['width']:
-                outputs.append("%s %s %s %s\n" % (
+                phys_size = ""
+                if output_params['width_mm']:
+                    # don't provide real values for privacy reasons - see
+                    # #1951 for details
+                    dpi = (int(output_params['width']) * 254 /
+                          int(output_params['width_mm']) / 10)
+                    if dpi > 300:
+                        dpi = 300
+                    elif dpi > 200:
+                        dpi = 200
+                    elif dpi > 150:
+                        dpi = 150
+                    else:
+                        # if lower, don't provide this info to the VM at all
+                        dpi = 0
+                    if dpi:
+                        # now calculate dimensions based on approximate DPI
+                        phys_size = " {} {}".format(
+                            int(output_params['width']) * 254 / dpi / 10,
+                            int(output_params['height']) * 254 / dpi / 10,
+                        )
+                outputs.append("%s %s %s %s%s\n" % (
                             output_params['width'],
                             output_params['height'],
                             output_params['x'],
-                            output_params['y']))
+                            output_params['y'],
+                            phys_size,
+                ))
     return outputs
 
 def notify_vm(vm, monitor_layout):