Simplify min/max computations in constructPath (bug 1135277)

- most of the time the current transform is a scaling one (modulo translation),
  hence it's possible to avoid to apply the transform on each bbox and then apply
  it a posteriori;
- compute the bbox when it's possible in the worker.

src/shared/util.js

@@ -720,6 +720,57 @@ class Util {
     return `#${hexNumbers[r]}${hexNumbers[g]}${hexNumbers[b]}`;
   }
 
+  // Apply a scaling matrix to some min/max values.
+  // If a scaling factor is negative then min and max must be
+  // swaped.
+  static scaleMinMax(transform, minMax) {
+    let temp;
+    if (transform[0]) {
+      if (transform[0] < 0) {
+        temp = minMax[0];
+        minMax[0] = minMax[1];
+        minMax[1] = temp;
+      }
+      minMax[0] *= transform[0];
+      minMax[1] *= transform[0];
+
+      if (transform[3] < 0) {
+        temp = minMax[2];
+        minMax[2] = minMax[3];
+        minMax[3] = temp;
+      }
+      minMax[2] *= transform[3];
+      minMax[3] *= transform[3];
+    } else {
+      temp = minMax[0];
+      minMax[0] = minMax[2];
+      minMax[2] = temp;
+      temp = minMax[1];
+      minMax[1] = minMax[3];
+      minMax[3] = temp;
+
+      if (transform[1] < 0) {
+        temp = minMax[2];
+        minMax[2] = minMax[3];
+        minMax[3] = temp;
+      }
+      minMax[2] *= transform[1];
+      minMax[3] *= transform[1];
+
+      if (transform[2] < 0) {
+        temp = minMax[0];
+        minMax[0] = minMax[1];
+        minMax[1] = temp;
+      }
+      minMax[0] *= transform[2];
+      minMax[1] *= transform[2];
+    }
+    minMax[0] += transform[4];
+    minMax[1] += transform[4];
+    minMax[2] += transform[5];
+    minMax[3] += transform[5];
+  }
+
   // Concatenates two transformation matrices together and returns the result.
   static transform(m1, m2) {
     return [