vk_swapchain: Use certain modes for unlocked

Uses mailbox, then immediate for unlocked framerate depending on
support for either. Also adds support for FIFO_RELAXED.

This function now assumes vsync_mode was originially configured to a value
that the driver supports.

vk_swapchain: ChooseSwapPresentMode determines updates

Simplifies swapchain a bit and allows us to change the present mode
during guest runtime.

vk_swapchain: Fix MSVC error

vk_swapchain: Enforce available present modes

Some frontends don't check the value of vsync_mode before comitting it.
Just as well, since a driver update or misconfiguration could problems
in the swap chain.

vk_swapchain: Silence warnings

Silences GCC warnings implicit-fallthrough and shadow, which apparently
are not enabled on clang.