[SDL] Bug in GL_SWAP_CONTROL extension name.

[Eyal Lotem]

The checked extension name is incorrect.
The attached patch fixes it to the correct name.

I don't think that under any setup, this extension name was ever
correct, which is why its not OR'd against the old check.

This fixes the "tear effect" (vsync problems) I had here with nVidia
GLX drivers.

Eyal
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