I came across a case where one pixel is almost transparent and the other
is fully transparent. Consider these pixels:

[ 1, 46, 250, 0 ]
[ 1, 42, 250, 4 ]

With our old logic, there was a color-delta of `1` between these pixels.
If we blend them both with white, we instead get a very low color-delta
value.

At first I was going to always blend with white, but I realized we are
using fully transparent as a signal that there is a gap/missing pixels
in either the before or after image. So I instead changed it to
"intentional transparency" where something like

[255, 255, 255, 0]
[0, 0, 0, 0]
[100, 100, 100, 0]

would be considered "intentional transparency". Of course, this
assumption can't always be made. But for us I think we can use it to get
rid of the edge-case that I present here. There's still a risk that
we'll encounter "wild" transparency with r===g===b but at least this is
better than what we have.

Some code we had was assuming that everything fully transparent was
intentionally added by us as filler for missing content on the right of
either the before or after image. To make things more explicit, I'm
using the term "filler pixel" here and setting it to always be 1,1,1,1.
This way the intentions in the code become clearer and at the same time
we have less risk of collisions with a transparent pixel that we do not
control.

This diff image looks slightly different now that we've made some
changes to the filler pixel.

Handle almost-transparent and transparent pixels better