[nongio]

Adds renderer::skia_vk, behind a new vulkan feature. It wraps a VkImage the host owns as a Skia render target with a top-left origin, and draws a scene into it, much as skia_fbo does for a GL framebuffer. lay-rs still loads no Vulkan itself: the host creates the instance, the device, the DirectContext and the image, and passes raw handles as Skia's own Vulkan types.

skia_fbo and the C renderer API now sit behind a gl feature. It is in the default set, so crates that keep default features see no change. gl-rs was only used by tests, so it moves to dev-dependencies. The unused GL direct_contexts import in drawing/scene.rs is gone. The engine is untouched.

tests/vulkan_image_cache.rs is a headless test of the GPU image-cache path on Vulkan. It uses ash, which is a dev-dependency only, to pick a device with a graphics queue and create a device-local RGBA8 image, then renders a scene with an image-cached layer through skia_vk. It checks that the cache surface was allocated on the Vulkan context and reused on the next frame, and that both the frame and a GPU subtree buffer match the raster render. Where no Vulkan device is available the test prints a note and returns.

Run it with:

cargo test --features vulkan --test vulkan_image_cache