Enabled SAMPLE_BUFFERS EGL attribute

This attribute is required for multisampling support.
Try to create Embedded context first

By mistake, this code would always create a desktop context. The correct
approach is to create an embedded (EGL) context and only fallback to
desktop if that doesn't work.
Cleaned up test code committed by mistake.
Merge branch 'modes'
Updated readme file
Merge branch 'master' of https://github.com/thefiddler/opentk
Merge branch 'master' of https://github.com/thefiddler/opentk into xamarin