Simplify workspace.getBlocksBoundingBox

Previously it returned x,y,width,height.  Returning top,bottom,left,right results in simpler code, both in this function and in downstream callers.

This commit makes the minumum change to the metrics_test.  I’m happy to change the test’s data if that makes more sense.

tests/jsunit/metrics_test.js

@@ -34,10 +34,10 @@ function makeMockWs(scale, x, y, width, height) {
   return {
     getBlocksBoundingBox: function() {
       return {
-        width: width,
-        height: height,
-        x: x,
-        y: y
+        top: y,
+        bottom: y + height,
+        left: x,
+        right: x + width
       }
     },
     scale: scale