Fix rendering of lines longer than 2^16

[A-Walrus]

Before things would be cast to u16 earlier than needed, which would cause problems for insanely long lines (longer than 2^16 ~ 65 thousand) In order to reproduce:

• Open file with really long lines, for example http://tom7.org/abc/paper.txt
• 65500l (go 65500 to the left)
• hold l for a bit

Before it would disappear when you reached 2^16 (65536), now it doesn't.

Note: currently moving around when the cursor is very far left is very slow, especially in debug mode, this is an unrelated issue.