Support SkEventTracer for profiling and debugging.

[fredizzimo]

Skia has a lot of internal trace calls, which eventually gets added to the SkEventTracer. In the C++ code, you can make your own instance of it, and call SetInstance, to do custom tracing and logging.

I'm adding some profiling to Neovide, but all the Skia internals are blackboxes, so it does not help that much. All I know is that with my current modifications, skia flush is occassionally taking a lot longer than usual, causing some frame drops.

The actual profiling is done through Tracy, and while the event tracer is not an exact match, I belive it would be possible to transform the events into something usable if it's exposed.

So I would be very happy if this could be supported.