Character composition messes up motion commands

[9viz]

Hi,

My only use case for vterm is Julia in which I have variables such as x̃ which is the character x followed by COMBINING TILDE. However, having this in the prompt in the vterm buffer seems to confuse the motion commands. To reproduce,

1. M-x vterm RET
2. Copy the xtilde thing above, and yank it in the vterm buffer
3. Now try inserting any other character and notice how the cursor position is off
4. Remove everything, and just say x, and repeat 3. You will observe that the cursor position is where you would expect.

With such composed characters inserted, it becomes a major chore to edit the text too!

I'm using vterm at commit 056ad74.