mesa: Add support for the GL_KHR_context_flush_control extension

mesa: Add support for the GL_KHR_context_flush_control extension

The GL side of this extension just provides an accessor via glGetIntegerv for
the value of GL_CONTEXT_RELEASE_BEHAVIOR so it is trivial to implement. There
is a constant on the context for the value of the enum which is initialised to
GL_CONTEXT_RELEASE_BEHAVIOR_FLUSH. The extension is always enabled because it
doesn't need any driver interaction to retrieve the value.

If the value of the enum is anything but FLUSH then _mesa_make_current will
now refrain from calling _mesa_flush. This should only affect drivers that
explicitly change the enum to a non-default value.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>