Reduce number of image style flushes on crop save

Problem/Motivation

I'm seeing the following when I update a single crop selection on an Image. Since the crop x,y coordinates have changed, ImageWidgetCropManager::updateCropProperties() is called. If we take a look at this method:

public function updateCropProperties(Crop $crop, array $crop_properties) {
  // Parse all properties if this crop have changed.
  foreach ($crop_properties as $crop_coordinate => $value) {
    // Edit the crop properties if he have changed.
    $crop->set($crop_coordinate, $value, TRUE)
    ->save();
  }
}

we can see $crop->set($crop_coordinate, $value, TRUE) is called for each item in $crop_properties.
Looking at $crop_properties, a sample value looks like:

Array
(
    [width] => 1055
    [height] => 593
    [x] => 528
    [y] => 1070
)

It appears that the Crop entity is being saved for each item in the array (so 4x). If we take a look at Crop::save():

Crop::save() {
    parent::save();
    image_path_flush($this->uri->value);
}

Thus for each Crop entity save, image_path_flush() is called. image_path_flush() looks like:

function image_path_flush($path) {
  $styles = ImageStyle::loadMultiple();
  foreach ($styles as $style) {
    $style->flush($path);
  }
}

In summary the side effect of saving the Crop entity for each setting of each of the 4 crop properties results in a load of all Image styles present on the system and a flush of these image styles.

Proposed resolution

When you have only a couple image styles and a couple of crop types, this probably isn't that noticeable. However, if you have more crop types and more image styles the impact of repeatedly flushing all of the image styles can reduce performance.

I'm not sure if it is necessary to save after setting each crop property. I think we can move the Crop::save() in updateCropProperties() to outside of the for loop.