How to use vulkan

[zoujiaqing]

[rubenvb]

There is currently no implementation of Vulkan support. Several things need to happen for that:

• Skia's Vulkan bits need to be built.
• Vulkan headers will probably be included next to the opengl-api in 3rdparty.
• skui::graphics will need to be extended with skia_vulkan_canvas and skia_vulkan_context.
• skui::gui will need to be extended with native_visual::vulkan.

In the future (as is the plan for the current OpenGL bits), the graphics API support will be mostly in a separate component, which might eventually be loaded at runtime by skui::gui.

[zoujiaqing]

thanks @rubenvb