Slow scrolling performance with ligatures disabled

[vdcow]

Issue description

I am using the latest version of neovim-qt built from master and I have slow scrolling performance for files with long enough lines (~150 - 200 symbols). But enabling the ligatures using GuiRenderLigatures resolves the issue - scrolling becomes fast.

Steps to reproduce

1. Open the file with long lines. E.g. you could generate it like this

for i in {1..20000}; do echo "Long line long line long line  long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line long line" >>test.txt; done

2. Scroll-down file to the bottom.
3. Return back to the beginning of the file, enable ligatures and scroll-down again.

Actual results

Scrolling with ligatures disabled looks slower and choppy. Scrolling time with ligatures disabled on my test is ~12 seconds when with enabling ligatures improves time to 9 seconds.

Expected results Scrolling with disabled ligatures should be as fast as with enabled ligatures

Additional details Used platform - Alt Linux Sisyphus

Note that on the test example only scrolling down and is slow. Scrolling up works just fine. Scrolling to the left/right is also laggy on the long lines when ligatures disabled.