Transparency removal

(This is a merge of a couple of issues)

**Is your feature request related to a problem? Please describe.**
Some images have transparency and the transparency is either undesired or the target image format cannot handle transparency. We can give the user more control how these cases are handled.

**Describe the solution you'd like**

Just like we have a section for preprocessing the size of an image, we could add a section to preprocess the transparency of an image, offering the following options (or more):

- Choose a background color (#347, #699)
- Texture Dilation (#735, [1](https://docs.substance3d.com/spdoc/padding-134643719.html), [2](http://wiki.polycount.com/wiki/Edge_padding))

The issue does not name files, tests, or an entry point. Start by reviewing the existing image-size preprocessing section and the linked texture-dilation references; done means users can choose how transparency is handled, including a background color or texture dilation, for formats that cannot preserve it.

Written by the indexing model from the issue text.