The indexed option of the PNG export in the SVG viewer has no effect.
This is because KEY_INDEXED is set to 256 while it seems to be expected
to be 8 if the number of palette entries should be 256 = 2^8.
A value of 256 will be ignored in
I'll post a simple patch, but the indexed option is still quite unusable
because it dithers a white background, even if only one other color is used.