cube: Rebuild swapchain only on a real size change

[perpetualbits]

On Wayland, handle_surface_configure() calls demo_resize() on every xdg_surface.configure. A compositor also sends configure for state changes — activation, maximization, tiling — and those usually carry the size the surface already has, so the swapchain and every image, view, framebuffer and command buffer are destroyed and recreated for nothing.

Locally a recreation costs about a millisecond and nobody notices. It becomes very visible wherever Vulkan calls are expensive. Under a compositor using focus-follows-mouse, moving the pointer across the window produces a configure per crossing, and each one froze the demo for roughly a second.

Measured

60 s per run, pointer crossing only, same machine and compositor both times:

	configures	swapchain rebuilds	visible stalls	worst stall
unpatched	178	92	9	1117 ms
patched	392 (391 same-size)	1	0	104 ms

The patched run also drew 4.3x as many frames in the same wall clock. Behaviour was checked natively first, where the change makes no visible difference.

Why this is safe

• !demo->swapchain_ready keeps the first configure creating the swapchain.
• The VK_ERROR_OUT_OF_DATE_KHR / VK_SUBOPTIMAL_KHR paths in draw() remain the safety net for anything that invalidates the swapchain without changing the size.
• This is how vkgears in mesa-demos has always handled it: record the size, compare, recreate only on a real difference.

Found while running vkcube over a remoting layer, where each Vulkan call is a network round trip and the cost of a needless rebuild is impossible to miss.

https://claude.ai/code/session_0153BQRRKAJcRSxUxRuEHQQs

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