Vulkan and new renderer related API changes
These are a bunch of changes to the public API about renderers and how we present it, culminating in enabling Vulkan by default.
To summarize:
- Include all the renderer headers in
<gsk/gsk.h>
- Remove all Vulkan-specific public API hidden behind ENABLE_VULKAN
- Deprecate existing Vulkan API
- Enable Vulkan by default