GTK: Offscreen pixel buffer fixes and several marshalling issues

[maxkatz6]

Fixes #56

1. GTK Offscreen rendering used wrong pixel format
2. GTK Offscreen wasn't rendered until first resize
3. Both offscreen and non-offscreen backend wasn't properly disposed, and it was reusing the same webview handle on re-attaching.
4. Several PInvokes were incorrectly defined (it was manually defined before AI era with static analysis, funny how much easier pinvokes are now)