brightness element

The brightness element is present for grayscale images with acceptable
dimension and compressed data size test results. It contains the following
parts:
- raw
- Raw contains a floating point number representing how bright the image gray levels get.
- score
- Score contains a floating point number between 0 and 10 (inclusive). This value is the Image Compliance estimate of the likelihood the image is bright enough. A 10 indicates the image passes the brightness test and is good. A 0 indicates the image fails this test and is too dim. Values in between 0 and 10 are a measure of the image brightness.