Scrollback performance is sluggish when many Emojis are displayed

[bwachter]

To reproduce do a find filling 2-3 pages, followed by cat test/emoji.txt. Scrolling over the emojis is annoying, and speed picks up as soon as it gets to the text section.

[rburchell]

I'm not sure there's much that can be done to improve this at our level, at least not without a lot of work. Simple profiling shows a lot of time spent in libpng/Qt/etc, which hints that there's some work to be done at improving emoji-heavy text.