Backends: Vulkan: fix present semaphore reuse for secondary viewports.

[Tom-Olsen]

What changed:
Keep image acquisition semaphores indexed by the rotating SemaphoreIndex, but select RenderCompleteSemaphore using the acquired swapchain image FrameIndex.

Reasoning:
A semaphore passed to vkQueuePresentKHR may remain in use until the corresponding swapchain image is acquired again. Rotating present wait semaphores independently can therefore trigger:

VUID-vkQueueSubmit-pSignalSemaphores-00067

This follows the Vulkan swapchain semaphore reuse guidance:
https://docs.vulkan.org/guide/latest/swapchain_semaphore_reuse.html

My setup:
Im using Linux (Fedora) with vulkan 1.4.341, sdl3, gcc 16.1.1, and ran with x11 as wayland doesnt support separate windwos due to lack of global mouse positioning if i understood correctly.

Error before fix:
[12:06:05] Ember: Validation layer ERROR: vkQueueSubmit(): pSubmits[0].pSignalSemaphores[0] (VkSemaphore 0x19930000001993) is being signaled by VkQueue 0x2b272940[Queue_Graphics], but it may still be in use by VkSwapchainKHR 0x19780000001978.
Most recently acquired image indices: 2, 1, [0], 2, 1, 2, 1.
(Brackets mark the last use of VkSemaphore 0x19930000001993 in a presentation operation.)
Swapchain image 0 was presented but was not re-acquired, so VkSemaphore 0x19930000001993 may still be in use and cannot be safely reused with image index 1.
Vulkan insight: See https://docs.vulkan.org/guide/latest/swapchain_semaphore_reuse.html for details on swapchain semaphore reuse. Examples of possible approaches:
a) Use a separate semaphore per swapchain image. Index these semaphores using the index of the acquired image.
b) Consider the VK_KHR_swapchain_maintenance1 extension. It allows using a VkFence with the presentation operation.
The Vulkan spec states: Each binary semaphore element of the pSignalSemaphores member of any element of pSubmits must be unsignaled when the semaphore signal operation it defines is executed on the device (https://docs.vulkan.org/spec/latest/chapters/cmdbuffers.html#VUID-vkQueueSubmit-pSignalSemaphores-00067)