RENDER_PNG=false RENDER_TILES=false still renders PNG

[tokee]

The logic for determining the need for a PNG is faulty, so specifying RENDER_PNG=false RENDER_TILES=false still forces a PNG. A workaround is to write VIPS_ONLY=true RENDER_PNG=false RENDER_TILES=false, but this should be fixed.

Reported by @thomasegense