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.

The problem is documented here:
http://www.adventuregamestudio.co.uk/wiki/Escaping_characters

See the examples labelled "aaa\\\\[bbb".