Delayed match-highlight causes bad partial reparses

[mtaran-google]

Repro steps:

1. Replace CodeMirror/mode/markdown/index.html with the attached index.html
2. Open that index.html
3. Paste in the contents of aaaa.txt (sorry I couldn't make up a small repro case)
4. In the console run editor.setCursor({line: 485, ch: 28, xRel: 1})

Expected:
Editor scrolls to the indicated position

Actual:
Editor scrolls to the indicated position, and then the highlighting gets messed up, flipping code-style and normal-style text. I'm pretty sure this is because of a reparse/redraw that doesn't start very far up above the upper edge of the viewport.

repro.zip

[marijnh]

The mode was, for some reason, keeping state in a variable rather than in its state object, and this kind of corruptions in rare situation are exactly what that causes. Attached patch fixes this.