To maintain maximum frame rate we render to the last acquired swapchain image but present the last but one drawn image. This behavior is optional through conditional compilation macros.
The transition of the swapchain image(s) can happen implicitly in the renderpass. This approach has been stated to be more efficient than using an explicit barrier.
See "Vulkan Programming Guide", Chapter 7: "Graphics Pipelines", section "Renderpasses".
Additional layer, extension and the callback itself are used/created when IMGUI_VULKAN_DEBUG_REPORT is defined. The callback reports seven (potential!) errors which will be fixed with another pull request.
Added a macro to switch between unlimited frame rate (VK_PRESENT_MODE_IMMEDIATE_KHR) and limited to 60 fps (VK_PRESENT_MODE_FIFO_KHR). Only the latter mode is guaranteed to be available, but the former one most likely is.
