Engine: Only escape "\[" once

It's not currently possible to display the sequence "\[" using a display
function, e.g. DrawStringWrapped. The engine consumes all \ before the
[, so that "\\\\\\\\\\\\\[" results in just "[" being displayed. This
corrects the escaping code for "\[" so that its behaviour is more
standard -- only replace the \[ once.
Merge pull request #172 from gurok/331_correct_escape_open_bracket

Engine: Only parse "\[" once