Make OCR settings selection image scale up less.

Previously, if you selected a long, thin row of pixels it would extend
past the end of your screen so you'd have to make another selection
before you could change any settings.
Setting the css to use height doesn't give you a huge zoom on small
selections like the previous setting did, but it's less disruptive.

web/index.css

@@ -163,7 +163,9 @@ video, canvas {
   color: white;
 }
 .canvasContextSelectionImage {
-  width: 100%;
+  height: 100%;
+  max-width:100%;
+  max-height:100%;
   display: block;
 }
 .welcome-card-wide.mdl-card {