vulkan: support pseudo-HDR10 mode via VK_EXT_hdr_metadata

This is not an optimal solution by any stretch of the imagination. Instead it's a hack, meant to work around what I believe to be a driver bug in the current implementations (affecting both AMD and Nvidia, apparently).

Basically, the idea is that for implementations which support VK_EXT_hdr_metadata but not VK_EXT_swapchain_colorspace, we can use the former to force the display into HDR10 mode. This means we should advertise support for such formats, on these platforms.

Closes #152 (closed)