Leaked graphics resources crash app when engine is disposed

[CedricGuillemet]

Linked to #464

Context:
Graphics resources are created and not disposed like VertexBuffer for example. Bgfx is shutdown when the engine is disposed. After some time, the GC tries to free the resources but because bgfx is not valid anymore, app crashes.

[bghgary]

@CedricGuillemet Is this still true?

[CedricGuillemet]

Hard to be sure without testing but I guess, yes. In the mean time, no user complained about this. I think we can close it.