libhb: fix availability of left/right only mixdowns.

[ghost]

Mixdowns introduced in 8d9969f

Availability was (probably accidentally) extended to supersets of Stereo input in e7b1636

    /*
     * The following mixdowns have a very specific purpose!!!
     *
     * Given 2-channel input, they allow discarding either of the right or
     * left channel to produce mono output instead of downmixing (the latter
     * may clip or lower volume depending on whether the downmix coefficients
     * are normalized or not). They are meant to be used for sources which
     * are known to actually be Mono (identical content in the L/R channels
     * or one of L/R is actually empty). They hardly make any sense when the
     * input has more than two channels.
     *
     * Thus specifically only allow them for Stereo input (layout == STEREO).
     */
    case HB_AMIXDOWN_LEFT:
    case HB_AMIXDOWN_RIGHT:
        return (layout == AV_CH_LAYOUT_STEREO);

[ghost]

@jstebbins I think I brought this up on IRC when I originally discovered it, but I cannot remember exactly what came of it (if you had a specific reason to enable the mixdowns for more-than-Stereo input or if you just asked me to fix it and I just never got around to it in, erm, about 5 years).

[ghost]

Added a commit to work around issue #3533 by forcing output samplerate to input samplerate.

(how) do we typically reference github issues in commit messages: should I use the full URL, just the issue number, or not mention it in the commit message at all?

[galad87]

The issue number is enough imho.