Lazy load trace surface preview

I would recommend testing this before merging.

In this patch, the trace tab's surface preview is not created until the first capture.  That helps with pages that resize the canvas *after* requesting a webgl context.  If it worked before hand, it was pure luck and a race condition.  I didn't try to have the minibar fill all possible width, though I could add that, but it didn't seem like that big of a deal.