Render changes in ZWE sequences

[joouha]

Hi Jonathan,

Currently if a zero-width escape sequence in a Screen changes, but the Char at the same position does not change, the new ZWE is not sent to the output. I think the expected behaviour is that all changes to ZWEs should be sent to the output.

I've read the warning about this being performance-critical code, and obviously checking for differences in ZWEs in addition to differences in the data buffer will take longer. However, by avoiding unnecessary calls to move_cursor if the cursor is already in the correct position, we can save a bit of time to make up for this.

My testing shows that this implementation is not significantly slower than the current implementation (and can be faster in some scenarios) - but I'd appreciate if someone could test it independently.

Let me know what you think,
Josiah