gpu downloaders don't work

Forcing the vulkan downloader (by commenting out the egl one), and forcing the cairo renderer, gives me this:

The debug spew says:

Creating dmabuf texture, format NV12:0x100000000000002,  premultiplied, 2 planes, memory format 7, downloader GskVulkanRenderer

and

Vulkan uploaded 1920x1080 NV12::0100000000000002 YUV dmabuf

The result with the ngl downloader aren't better. There's a critical warning about toggle refs interfering with each other, because we are attaching two different textures to the same image. Once I patch that up to swap out the toggle reference, I just get black.

Comparing what the gpu downloaders do to the code in the old GL renderer, the difference is that the old GL renderer imports the dmabuf (possibly as an external texture), then blits the contents to an offscreen, and turns the offscreen into a GL texture to download.

The gpu downloaders import the dmabuf (possible as an external image), then take the texture ID of that image and wrap it in a GL texture to download. Whats missing is the blit in between.