monitor: Set MINIMUM_LOGICAL_HEIGHT to 480

Using 800x600 as minimum logical size is very 4:3 thinking, while a lot of
modern devices are 16:9. The specific reason for this commit is to allow
1.5 scaling at mini-laptops (clamshell devices) with e.g. a 5.5"
1280x720 screen. Given that this device has a keyboard, one obviously
is not holding it very close to ones eyes and at 220 dpi that means the text
is too small at scale 1.0. For one real world example of such a device see:
https://en.wikipedia.org/wiki/GPD_Win

https://bugzilla.gnome.org/show_bug.cgi?id=792765

src/backends/meta-monitor.c

@@ -34,7 +34,7 @@
 #define MINIMUM_SCALE_FACTOR 1.0f
 #define MAXIMUM_SCALE_FACTOR 4.0f
 #define MINIMUM_LOGICAL_WIDTH 800
-#define MINIMUM_LOGICAL_HEIGHT 600
+#define MINIMUM_LOGICAL_HEIGHT 480
 #define MAXIMUM_REFRESH_RATE_DIFF 0.001
 
 typedef struct _MetaMonitorMode