Support rowgap not divisible by 4

[joeha480]

Row spacing not divisible by 4 is not supported by the existing implementations.

[joeha480]

Supporting rowgaps in Index embossers will require an update to the embosser firmware.

The graphics modes are deprecated according to Index. This leaves two options: "transparent mode" and "floating-page". Because floating-page is more complicated to implement and because it doesn't support duplex (at the moment), transparent mode is the better option. However, it too requires a firmware update: support for 2.5 mm line spacing. Having a 2.5 mm line spacing enables a kind of graphics mode which works well with PEF, since the height of the page is defined in 8-dot cell heights. Also, setting line spacing in graphics mode is already implemented in Braille Utils, making it a smaller change to support it. Furthermore, floating-page may have speed implications.

I will revisit this issue once the new firmware is available.