Allow removal of actual & expected labels in delta images

[SimonMarquis]

As mentioned in #1465 (comment), I was wondering if there is a way to remove the actual & expected labels on the generated delta image?
For us, this does not provide any useful information since we are used to have before/after images next to each other, and if the changed pixels were located below those label, they would hide it.
I feel like "left == before" & and "right == after" is a common convention, but that might be true only for us using LTR locales.

The boolean ApngVerifier#withErrorText seems to be be related to this, but it is not exposed as a public API (and always true in the default configuration).