consider allowing custom X positioning of display characters

[Lx]

The hardware-based right justification is known to be buggy.

Given the existence of 1px-wide spaces (as discussed in #1), it is possible to fake right-justification (or indeed draw characters at any X position and with any desired spacing).

I prepared the following kludge to fix the right justification in a production environment:

_DISPLAY_WIDTH = 120
_CHARS_BY_WIDTH = {
    3: b'.',
    4: b'I1: ',
    5: b'0-',
    6: b'ABCDEFGHJKLMNOPQRSTUVWXYZ23456789',
}
_WIDTHS_BY_CHAR = {}
for width, chars in _CHARS_BY_WIDTH.items():
    for char in chars:
        _WIDTHS_BY_CHAR[char] = width


def pixel_width(string):
    width = 0
    for char in string:
        if char not in _WIDTHS_BY_CHAR:
            raise ValueError('unknown width for character "%s"' % char)
        width += _WIDTHS_BY_CHAR[char]
    return width


def fix_right_justification(bytestring):
    if rb'\R' not in bytestring:
        return bytestring
    left_start = 7
    left_end = bytestring.index(rb'\R', left_start)
    right_start = left_end + len(rb'\R')
    right_end = bytestring.index(
        b'\n', right_start
    ) if b'\n' in bytestring[right_start:] else bytestring.index(
        b'\r', right_start)
    left = bytestring[left_start:left_end]
    right = bytestring[right_start:right_end]
    left_width = pixel_width(left)
    right_width = pixel_width(right)
    padding_width = _DISPLAY_WIDTH - 2 - left_width - right_width
    padding = b''
    space_width = _WIDTHS_BY_CHAR[ord(' ')]
    while padding_width >= space_width:
        padding += b' '
        padding_width -= space_width
    while padding_width > 0:
        padding += b'\xff'
        padding_width -= 1
    return bytestring[:left_end] + padding + bytestring[right_start:]

fix_right_justification takes bytes as returned by DisplayMessage.to_bytes() and further modifies them.

Think about how best this could be incorporated into the library.