Take snapshots of formulas rendered by LaTeX and compute visual diffs

The same test cases we use for our screenshots from Firefox are now also
being rendered by pdflatex, so the resulting images can be used as reference
for how things are supposed to look (if we concentrate on compatibility with
LaTeX).  To make comparisons even easier, the differences between LaTeX and
Firefox snapshots are rendered in a visual way, using different colors.

Discussed in pull request #268.

dockers/texcmp/Dockerfile

@@ -0,0 +1,11 @@
+# convert from PDF to PNG with -flatten looks really bad on 14.04 LTS
+FROM ubuntu:15.04
+
+MAINTAINER Martin von Gagern <gagern@ma.tum.de>
+
+# Disable regular updates, but keep security updates
+RUN sed -i 's/^\(deb.*updates\)/#\1/' /etc/apt/sources.list && apt-get update
+
+# Install all required packages, but try not to pull in TOO much
+RUN apt-get -qy --no-install-recommends install \
+  texlive-latex-base etoolbox imagemagick ghostscript nodejs