* Texture support (#217)

* Feature/event loop interop (#218)

* Feature/callback messages (#220)

* fix: remove initial WaitEvent

now that we have implemented the proper event loop.
Delete the first WaitEvent

* lazy initialize go-gl/gl on first texture frame

* align BinaryMessenger with *-channels

BinaryMessenger:
SendNoReply -> Send
Send -> SendWithReply

* change the priorityQueue to specifically contain embedder.FlutterTask's

* Adds comments and renames internals. Also some re-ordering of init code. (#235)

* Addressing comments & fix pointer warning

* implement RunOnCurrentThread check using C call

* Custom error codes on the plugin side (#238)

Custom error codes on the plugin side

* rely on LockOSThread to ensure engine task are run on the same thread