Convert all types to enums

[mrobinson]

This is a meta bug tracking the enum conversion part of #370. Not only will this allow removing Multi, but it opens up the possibility of runtime support for both WGL and ANGLE and also ANGLE on macOS platforms (very important for the future where OpenGL is deprecated on Apple platforms).

• Adapter (Make Adapter an enum #385)
• ContextDescriptor (Make ContextDescriptor an enum #398)
• Context (Convert Context into an enum #399)
• Surface and SurfaceTexture (Convert Surface and SurfaceTexture into enums #400)
• Device (this will require temporarily removing Device::connection)
• Connection