Include bindVertexBuffer in Context.setDirty()

[davidmanzanares]

While doing a PoC of integrating CARTO VL with Mapbox GL using this new API we found a flickering issue (the screen was cleared to white intermittently sometimes).

The problem only happened when using WebGL outside the rendering loop (async).

We discovered that BindVertexBuffer.set() was being called after our async code executed and that the if (this.current !== v || this.dirty === true) { didn't pass in the first time set was called after our code, which made our buffers bound and MGL buffers unbound, causing the problems.

Adding this.bindVertexBuffer.dirty = true; to Context.setDirty() seems to fix the issue.

Thank you!

[ansis]

Thanks @davidmanzanares! looks like index buffers aren't handled correctly either but I'll push a fix for that directly to the pr.