[OpenTK] Removed unused GraphicsMode field
[OpenTK] Clamp max AA level to 64x

This protects against a potential denial-of-service issue during mode
selection, where the platform backend will try to reduce AA level until
a supported mode is found.
[OpenTK] Allow a GraphicsMode.Buffers value of 0

0 in this case means “don’t care”.
[OpenTK] Moved RelaxGraphicsMode to Utilities

This functionality is useful for all platform backends and is not
specific to the X11 backend.
[Win] Use RelaxGraphicsMode to find optimal mode
[Max] Use RelaxGraphicsMode to find optimal mode
[SDL2] Use RelaxGraphicsMode to find optimal mode

For SDL2, RelaxGraphicsMode requires us to clear context attributes
between consecutive attempts. This is implemented by calling
ClearGLAttributes().
[OpenTK] Don't skip AA modes in RelaxGraphicsMode

Previously, specifying an odd AA mode (e.g. 5x) would have been relaxed
to 0x. Now, it will be correctly relaxed to 4x.
Merge branch 'graphicsmode' into develop