Whether a WebGL2 context exists or can be created. Probes at most once.
True between a context loss and its restoration: frames go through 2D meanwhile.
Draw a batch into the top-left widthPx by heightPx of the surface,
cleared first. Returns false when the context is lost or absent, in
which case nothing was drawn and the caller must not blit.
One WebGL2 context, its program and its buffers, shared by every backend built for the same surface factory. Reference counted by mounted backends so the drawing buffer is given back once the last chart is destroyed; the context itself is kept, because a canvas whose context has been lost on purpose hands back that same lost context on the next
getContext.