To support High-DPI displays, generate images at 4 times their displayed
resolution, which should be enough to support all current devices
according to https://material.io/devices/.
* Support CRLF when splitting code lines for display
* refactor, fix mixed match
* fmt
* split on both LF and CRLF, use raw literals in regexes
* simplify
Tests indicate that line length alone does not make browsers slow, so
increase the default threshold after which diffs get surpressed for line
length from 500 to a more reasonable 5000 characters.
Fixes: https://github.com/go-gitea/gitea/issues/1826