add new tiles right at the "end" of the existing tiles instead of interspersing them
change output file name to game name
if user loads a file, the output file should have the same name (possibly with a suffix)
separate colour-mapping from bitsy palette?
idea: copy the "new palette" colour inputs above the palette selection. add a default option to the palette selection called "as above". leave the "new palette" option as is.
if bg/fg are a part of SelectedPalette they will need to be separated out in the application state.
SelectedPalette will need a new enum variant.
render_preview() will need a new step to map the background/foreground to the desired colours, if selected palette is not "as above".