(feat): add built-in option to clone the canvas

- this is almost always needed by developers, so figured I should add
  it into the library itself (well again, but w/o a node-canvas dep)
  - while it should probably be the default, will introduce it as
    opt-in so it's not a breaking change

Author: agilgur5

index.es6

@@ -1,4 +1,5 @@
-export default function trimCanvas (canvas) {
+export default function trimCanvas (canvas, options = {}) {
+  const {clone} = options
   const context = canvas.getContext('2d')
 
   const imgWidth = canvas.width
@@ -21,6 +22,15 @@ export default function trimCanvas (canvas) {
   const trimmedData = context.getImageData(cropLeft, cropTop, cropXDiff,
     cropYDiff)
 
+  // early return if cloning
+  if (clone) {
+    const copy = document.createElement('canvas')
+    copy.width = cropXDiff
+    copy.height = cropYDiff
+    copy.getContext('2d').putImageData(trimmedData, 0, 0)
+    return copy
+  }
+
   // set the trimmed width and height
   canvas.width = cropXDiff
   canvas.height = cropYDiff