Apply ansi color only on newly added region of the buffer.

From docs about `compilation-filter-hook`:
>Hook run after `compilation-filter` has inserted a string into the buffer.
> is called with the variable `compilation-filter-start` bound
>to the position of the start of the inserted text, and point at
>its end.

I have no numbers to back it up. But subjectively output on my machine appears a bit faster.