Sdl2InputDriver.Dispose() would call SDL_DelEventWatch with a different
"user_data" parameter than SDL_AdEventWatch. This caused the EventFilter
to remain registered and subsequently crash when closing and reopening a
window.
SDL_GL_GetCurrentContext will allow us to replace the weird
implementation in Sdl2Factory.CreateGetCurrentGraphicsContext()
and the latter to disable mouse emulation if we wish to.
Conflicts:
Source/OpenTK/Platform/SDL2/Sdl2.cs
SDL_GL_GetDrawableSize was introduced after 2.0.0, so we need to check for that version before we call this method. Fixes MethodNotFounException on vanilla libsdl2-2.0.0 on linux.
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