#152 add beforeRender event

[mdamien]

Trigger a beforeRender event for each renderer. Because the events are synchrones, it acts like a hook.

I can be useful in many cases:

• tuning the settings for each render (disabling a plugin while moving the camera for example)
• timing a renderer (to display a FPS counter for each renderer for example)

[sim51]

Thanks for this PR, even though it hasn't been merged.

For a few years, we focused on Sigma.js v2, and this week we released it!
It is probably better now to upgrade to this new version as v1 will no longer be maintained.

This PR will therefore be closed.