Fixes #8917

Description of Change

All I've done is to make sure the code uses the byte bounds instead of the pixel bounds when blacking out the image.

Issue

Previously, the code would just stop processing after a quarter. An example with a 3x3 image gives:

w = 3
h = 3

Such that we cover pixel space indices [0,9).

However, we would then increment the index variable by 4 as if we were in byte space, terminating after 25% of the processing.

Bonus

I also took the liberty of making the comments more concise.