Do not run embedding code in the same thread as the compositor code

[paulrouget]

GL operations need to run in the main thread on Desktop, and on a the GL thread on mobile.

If i'm not mistaken, as of today, the GL code runs in the thread used to start Servo. In other words, the embedding glue code runs in the compositor thread.

This means, for example on Android, that any communication with Servo (even non-compositor related operations, like navigating the history) need to happen in the GL thread.

We need to be able to start Servo from any thread, and have a mechanism to start the compositor in the proper thread.

[paulrouget]

The goal here would be to let the embedder starts the libservo::Servo in an any thread.

The libservo code will need to know in which thread to start compositor code. So the emebdder needs to provide a way to run code in the desired thread.

Maybe just providing a callback run_in_gl_thread might do it.