Image focal point UI is invisible on light images.

[abea]

To Reproduce Steps to reproduce the behavior:

1. Edit a piece or widget with an images singleton that has the focal point enabled.
2. Upload a light (mostly white) image and try to set a focal point.

Expected behavior You should be able to (reasonably) clearly see where the focal point was placed.

Describe the bug The focal point indicator is white and partially transparent, so it's not visible on very light images.

[boutell]

cc @stuartromanek