glxgears: Support GLX_EXT_swap_control for querying the current swap interval.
glxgears: Support GLX_EXT_swap_control for querying the current swap interval.

Signed-off-by: Aaron Plattner <7c39395e2e801146cdbd7fb4a41282e4f38db44e@nvidia.com>
Signed-off-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

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