Add a new variable, EPS2PNG_RES, to set the resolution eps2png uses to
convert graphics. Change the default from 82 to 100, for clearer images.

PR: docs/28237
Submitted by: G. Adam Stanislav <adam@whizkidtech.net>

While I'm here, include some suffix rules to convert .scr files to .png
files. Nothing uses these, yet, but should do shortly.