This ensures both that we signal a semaphore for a dmabuf when we export
an image and that we import semaphores for dmabufs and wait on them.
Fixes Vulkan node-editor displaying the Vulkan renderer in the sidebar.
This adds GSK_GPU_IMAGE_CAN_MIPMAP and GSK_GPU_IMAGE_MIPMAP flags and
support to ensure_image() and image creation functions for creating a
mipmapped image.
Mipmaps are created using the new mipmap op that uses
glGenerateMipmap() on GL and equivalent blit ops on Vulkan.
This is then used to ensure the image is mipmapped when rendering it
with a texture-scale node.
