Improve scaling on small phone screens.

... at the expense of pixel perfection. :(

game.css

@@ -13,9 +13,24 @@ body {
     height: 100vh;
 }
 
-canvas {
-    display: block;
-    margin: auto auto;
+/* Scale up canvas image on small screens.  Loses pixel perfectness
+   but improves visibility on phones. */
+@media (max-width: 960px) or (max-height: 720px) {
+    canvas {
+        display: block;
+        margin: auto auto;
+        image-rendering: pixelated;
+        object-fit: contain;
+        width: 100%;
+        height: 100%;
+    }
+}
+
+@media (min-width: 960px) or (min-height: 720px) {
+    canvas {
+        display: block;
+        margin: auto auto;
+    }
 }
 
 @media (pointer: fine) {