input-method-v1: track/render popups and adapt virtual keyboard integration for new Smithay API

[WantenMN]

Summary

This PR adds end-to-end handling for input-method-v1 popups (IME candidate/panel surfaces) and updates niri's virtual keyboard integration to match current Smithay behavior.

What changed

• Added InputMethodV1ManagerState initialization and storage in compositor state.
• Implemented InputMethodV1Handler in State:
  • track popup lifecycle (new_popup, popup_repositioned, dismiss_popup),
  • update popup scale/transform from the target output,
  • compute parent geometry through window/layout context.
• Added rendering of tracked input-method-v1 popup surfaces in the output render pass.
• Added commit-time redraw handling for input-method-v1 popup commits.
• Simplified protocols/virtual_keyboard.rs to delegate to Smithay's current virtual keyboard manager flow.

Why

With the current Smithay stack, niri needs explicit input-method-v1 popup management to correctly place and render IME candidate surfaces and to ensure redraws happen when those surfaces commit.

In parallel, virtual keyboard request handling is now provided inside Smithay, so the previous custom event bridging in niri is no longer the right integration point.

Merge dependency

This PR depends on Smithay changes and should only be merged after:

• feat: input-method-v1 Smithay/smithay#1950

[Atan-D-RP4]

Does this resolve #3274?

[Sempyos]

I would note.
Smithay/smithay#1937