Support GL video textures as webgl textures

[jdm]

#24233 added support for getting pixel data from a video frame and using that as a webgl texture, but creates an empty frame when we're using GL textures instead of raw pixels.

Yes, I think adding a new code path where we send a texture ID to the WebGL thread and use something like glBlitFramebuffer or glCopySubTexImage2D to copy the texture data to the destination texture will make the most sense.

[jdm]

master...ferjm:webgl.video.gl is the WIP branch.

[ceyusa]

Not sure where to comment about this. I'll just bring a comment I put in the branch:

I have just tested the old implementation (x11/raw frames) and it doesn't work anymore. Perhaps the surfman change produced a regression?

And I can't enable WebGL in Linux/Wayland now with surfman (i knew how to enable it in the old offscreen-rendering-context, but not anymore :/).