Vulkan: Control secondary viewports format and present mode

API changes:
- Secondary viewports surface format and present mode can now be controlled by the application view ImGui_ImplVulkan_RequestSecondaryViewportsChanges(...)
Impl changes (not exposed to the user):
- Each viewport now has its own VkRenderPass and VkPipeline (cached in the backed data). This fits to the fact that each viewport might have a different format (the previous code was making this (quite reasonable) assumption).
- Recycle some viewport resource on swapchain recreation if possible (CommandPool, CommandBuffer, Fence, Semaphores) rather than recreating them every time.