[dvipdfmx] Default resolution 600dpi cannot be changed

[Igor Liferenko]

> So, resolution passed to mktexpk (600) does not match the one
> specified in argument (300).

Also, notice the following fact:

if we use default mktexpk script, but in the beginning of it add this line:

    set -- --mfmode / --bdpi 300 --mag 1+0/300 --dpi 300 cmr10

then dvipdfmx uses cmr10.300pk which mktexpk generates.