[ft] Different font appearance depending on matrix

[Jan Engelhardt]

Depending on how one chooses font scaling, the result looks different.
As a baseline, I am using

[1] xterm -fa 'DejaVu Sans Mono:size=26:matrix=0.85 0 0 1'

this can be rewritten to

[2] xterm -fa 'DejaVu Sans Mono:size=13:matrix=1.7 0 0 2'
[3] xterm -fa 'DejaVu Sans Mono:size=7.5:matrix=3.4 0 0 4'
[4] xterm -fa 'DejaVu Sans Mono:size=52:matrix=0.425 0 0 0.5'

[5] xterm -fa 'DejaVu Sans Mono:size=22.1:matrix=1 0 0 1.17647'

The font looks slightly different for these four configurations (and
in case of [3] even causes a different window size) - screenshot at
http://inai.de/files/ftmatrix.png .

What may be the reason behind the different renderings?
There might be rounding erros in cases like [5], but [1]/[2]?