Draw a triangle #230
Description
Make a black box. Put three vertices into that box. Put a color on each vertex. Interpolate the colors between the vertices to draw a triangle in the box.
This is much easier with GL than without it. A possible path is to emulate libdrm and go through the kernel to get a GPU handle, and then submit drawing requests on that handle.
The triangle does not have to be drawn on-screen. For that, see #231.
Note that using the existing lib/shapes functionality does not count. We would like to be able to offload the drawing from the vat, so that we can wait for it like typical I/O.