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, 2)