text-input: cursor rectangle is not synchronized with wl_surface

Making a new issue for #39 (comment 847212). Changing the cursor rectangle can affect the window placed near it.

If:

• Text is typed, changing the cursor position
• The client updates the cursor position, then commits the text-input
• The client then updates the surface contents, then commits the wl_surface

There's a gap between the second and third steps where the compositor could render a frame with the overlaid window in the wrong position.