Cache translateBufferLineToString results for viewport

[Tyriar]

Forked from discussion in https://github.com/xtermjs/xterm.js/issues/1833

Proposal:

• Cache results of translateBufferLineToString when it's called for a row within the viewport
• Discard when:
  • The line changes (perhaps just when anything changes?)
  • The line is no longer in the viewport

This would ensure we're not duplicating calls to translateBufferLineToString for the hot section (viewport) while not blowing out memory by doing this for the entire buffer.

[jerch]

Ah sweet, I like the discard rules and limiting it to the viewport. It would at least deal with the linkifier and the lineFeed consumer well.