A while ago, I calculated some bounds on the intermediate values inside
the self-guided filter. These bounds turned out to be not quite correct
in one particular instance (when we have a large region of max-value
pixels).

This caused a variable to overflow a uint32_t when decoding 12-bit
streams in the reference decoder, and would force 8/10-bit-only
hardware to use wider buffers than intended in order to match the
reference code.

Fortunately, this can be fixed quite easily, with minimal changes
to the filter output. See comments within the patch for the exact
details.

Also re-instate a Wikipedia link which seems to have gone missing
but which provided useful context for the derivation of the bounds.

Change-Id: I83d4a277a37eff048af9989cccf19202fafb17b5