fix: check for NULL view->container in constrain_popup to prevent segfault

[xieguaiwu]

When using an IME (Input Method Editor, e.g. fcitx, ibus, anthywl) with sway, closing a window while the IME candidate popup is still active can cause sway to crash with a segfault.

Root Cause

The crash occurs in constrain_popup() in sway/input/text_input.c:

1. A view is unmapped and its container is freed (set to NULL)
2. The IME popup still holds a reference to the destroyed view via popup->desc.view
3. When the IME commits a new frame for the popup, handle_im_popup_surface_commit() calls constrain_popup()
4. constrain_popup() accesses view->container->pending.content_x without checking if view->container is non-NULL → NULL pointer dereference → segfault

The Fix

Add a NULL guard for view->container before dereferencing it. If the container is NULL, the view is no longer valid and we return early rather than crashing.

Reproduction

1. Configure sway with an IME (e.g., anthywl)
2. Open any application with a text input (e.g., gedit, Firefox)
3. Activate the IME so the candidate popup appears
4. Close the window (e.g., via keyboard shortcut)
5. Sway crashes with SIGSEGV in constrain_popup

Backtrace

#0  constrain_popup (popup=0x...) at ../sway-1.10/sway/input/text_input.c:159
#1  0x... in handle_im_popup_surface_commit (...)
#2  0x... in wl_signal_emit_mutable (wayland-server.c)
#3  0x... in surface_commit_state (wlroots)

Fixes #8541

[emersion]

This PR contains many unrelated commits.

[xieguaiwu]

Branch has been rebased onto the latest upstream master. The history is now a single clean commit — no more unrelated commits.

/cc @emersion