hwcontext_vulkan: dlopen libvulkan

hwcontext_vulkan: dlopen libvulkan

While Vulkan itself went more or less the way it was expected to go,
libvulkan didn't quite solve all of the opengl loader issues. It's multi-vendor,
yes, but unfortunately, the code is Google/Khronos QUALITY, so suffers from
big static linking issues (static linking on anything but OSX is unsupported),
has bugs, and due to the prefix system used, there are 3 or so ways to type out
functions.

Just solve all of those problems by dlopening it. We even have nice emulation
for it on Windows.