some systems need segments to be letterboxed to some overarching res

[vadosnaprimer]

• target resolution is deduced automatically by looking at all segments
• since we can letterbox at native res during initial import, resizing is not involved
• resize based MS import is incompatible with this setting

msLetterbox is the variable that enables this, has to be set in addition to ms.

fixed #18

[vadosnaprimer]

Tested on https://tasvideos.org/4829M, seems to work fine.

[despoa]

Few observations and suggestions:

• If the sample parameter in msLetterbox isn't going to be used at all, it should be removed, unless Avisynth syntax requires it.
• In encode.avs, lines 60 and 61 should be right after line 53 to make it clear to the end user that multisegment letterboxing is an option.
• In the same file, msAudio will be completely ignored and will not work if msLetterbox is true. Here's a possible fix.

if (ms && !msLetterbox) {
	resized = resized \
		.AppendSegment(msBase, msStart, msEnd, msFormat, resizer, hd, pixelType) \
		.ConvertToRGB32()
}

if ( (ms || msLetterbox) && msAudio) {
	resized = AudioDub(resized, WavSource(msAudiotrack))
}

[vadosnaprimer]

Check.

[despoa]

Looks good.

[fsvgm777]

Works as intended.