Export indexed spritesheets w/ opaque bg color

Hey, small thing: it could be nice to have a box to tick in the spritesheet export windows in order to have a “background” instead of a “layer” for the generated spritesheet.

It would avoid to have a transparent background when exporting in index mode, if you do not want it.

Logically, such a box should be located in the “output” tab of the spritesheet export window.