Add "rasterise" / "rasterisation" to UK dictionary

[DimitriPapadopoulos]

US counterparts "rasterize" / "rasterization" are included in the en_US-large aspell dictionary.

From the Google Books Ngram database:

• https://books.google.com/ngrams/graph?content=rasterise%2Crasterize&year_start=1800&year_end=2019
• https://books.google.com/ngrams/graph?content=rasterisation%2Crasterization&year_start=1800&year_end=2019
• http://app.aspell.net/lookup-freq?words=rasterise%0D%0Arasterize%0D%0Arasterisation%0D%0Arasterization;also=similar;parm=full