Make buffer length equal rows + scrollback, make alt buffer always 0 scrollback

[Tyriar]

Fixes #847
Fixes #802

[coveralls]

Coverage increased (+0.3%) to 71.585% when pulling e7ee3ec on Tyriar:847_proper_scrollback_value into 687a5e2 on sourcelair:v3.

[parisk]

My main concern with this PR is that a buffer becomes aware if it is the alt buffer or not.

Instead of doing this, I propose setting the length of each buffer explicitly via the BufferSet which is aware of both the normal and the alt buffer, and essentially manages them.

[Tyriar]

@parisk I'm not sure that's better though, right now the "alt buffer scrollback is 0" logic is contained in a single place (as well as the comment explaining it). If we had BufferSet manage it, then we would need a scrollback parameter on the Buffer.constructor, Buffer.clear and Buffer.resize.

If preferable we could change the _isAltBuffer property to something like _hasScrollback to be more generic?

[coveralls]

Coverage increased (+0.3%) to 71.825% when pulling 996eb1b on Tyriar:847_proper_scrollback_value into 91bf319 on sourcelair:v3.

[Tyriar]

Ping @parisk

[parisk]

we would need a scrollback parameter on the Buffer.constructor, Buffer.clear and Buffer.resize

Couldn't we just pass it only in the constructor and have clear and resize sourcing it from the buffer's attributes?

Renaming _isAltBuffer to _hasScrollback is 👌 too.

[Tyriar]

@parisk but they change and the values are needed every time these functions are called. We would either have to pipe through the scrollback (or total height) every time from BufferSet or just have Buffer manage it itself.

[coveralls]

Coverage increased (+0.3%) to 71.89% when pulling 9dd1083 on Tyriar:847_proper_scrollback_value into aa438c8 on sourcelair:v3.

[coveralls]

Coverage increased (+0.3%) to 71.89% when pulling 7b0d0e8 on Tyriar:847_proper_scrollback_value into aa438c8 on sourcelair:v3.