DPI setting works only for png files

[hnesk]

Remove it or implement for other image types, too

browse-ocrd/ocrd_browser/model/document.py

Line 230 in d482ac9

image_bytes = add_dpi_to_png_buffer(image_array.tostring(), dpi)

[bertsky]

IIUC the PNGs created by ocrd_browser are only for ad-hoc viewing. So how can we utilise or show the (original) image DPI?