darkroom: fix viewport not refreshing on Win32 GDK backend

[jsmucr]

On Windows (GTK 3.24.52, MSYS2/UCRT64), the darkroom viewport failed to redraw after zoom, pan, or overlay changes. The widget's draw handler was called but the content never appeared on screen until an external trigger (e.g. window move) forced a full surface recreation.

Root cause: commit 76ff82f ("fix regression in darkroom switching between images") introduced a cairo_surface_reference() to cairo_get_target(cri) — GTK's internal drawing surface — to cache the rendered image for smooth transitions when loading_screen is OFF. On the Win32 GDK backend this prevents GTK from properly invalidating the widget, causing gtk_widget_queue_draw() to silently fail.

The original double buffering via a standalone gui->surface was removed in 8976447 ("remove unnecessary double buffering"). Commit 76ff82f re-introduced surface caching but took a shortcut by referencing GTK's target rather than maintaining a proper standalone surface.

Fix by copying the rendered content into a standalone cairo_image_surface instead of referencing GTK's internal target. Also:

• Add pipe->changed to _full_request() so the pipe is resubmitted when zoom/pan changes are pending, regardless of pipe status.
• Add !pipe->loading to expose_full condition to prevent displaying stale backbuf data while a new image is being loaded.
• Clear backbuf, output_imgid, and cached surface in leave() so re-entering darkroom starts with a clean state.
• Paint darkroom background when no cached surface is available (first entry from lighttable with loading_screen OFF).

Note: GTK overlay widgets (e.g. toast messages) may still composite against a stale backing store during the loading phase. This is a pre-existing GTK Win32 compositor limitation and is not addressed here.

Fixes #20831

[jenshannoschwalm]

I don't have access to windows systems but from your description i think we might have fixed this in master

[jsmucr]

I'll check.

[jsmucr]

@jenshannoschwalm No, still not fixed, I'm afraid.