Fix negative dimensions in scene rect sizing

[beeblebrox]

Summary

• Fix crash when dragging a tiled window across multiple outputs without dropping on intermediate output
• wlr_scene_rect_set_size assertion width >= 0 && height >= 0 fails at types/scene/wlr_scene.c:739
• Same class of bug as Crash with titlebar_padding 1 10000 #8120 (fixed by 62fd8c4 for height, but width was missed)

Root Cause

During cross-output tiling drags, box dimensions can go negative via:

1. resize_box() subtracting thickness * 2 from width/height (WLR_EDGE_NONE case)
2. Threshold calculations producing negative values during cross-output coordinate transforms
3. Border width passed unclamped to wlr_scene_rect_set_size in arrange_container()

Changes

• sway/input/seatop_move_tiling.c: Clamp dimensions in resize_box() and update_indicator()
• sway/desktop/transaction.c: Clamp border width in arrange_container() (matches existing height clamp from 62fd8c4)
• sway/tree/container.c: Clamp pixman region box dimensions in update_rect_list()

Environment

• Sway 1.11, wlroots 0.19.2, Mesa 26.0.2, Linux 6.19.7
• 3 monitors in non-trivial layout (mix of side-by-side and stacked)
• Reproducible: drag tiled window across outputs without releasing, particularly with a gamescope application on the intermediate monitor

Stack Trace

wlr_scene.c:739: wlr_scene_rect_set_size: Assertion `width >= 0 && height >= 0' failed.
#4  wlr_scene_rect_set_size (libwlroots-0.19.so + 0x3e0de)
#5  sway+0x3b6e4
#6  wl_signal_emit_mutable (libwayland-server.so.0) x2
#7  libwlroots-0.19.so + 0x764b9
#8  wl_event_loop_dispatch → wl_display_run

Test Plan

• Drag tiled window across 3 outputs without dropping on intermediate
• Verify crash no longer occurs
• Drag windows between all monitor pairs
• Create many small tiled containers and drag across outputs
• Resize windows to minimum near output boundaries