Fix xcompositeCreateRegionFromBorderClip return type

The `XCompositeCreateRegionFromBorderClip()` function returns an Xfixes region (an XID), not an Xlib region, so a wrapper function and `makeRegion` invocation are unnecessary.

The FFI definition for this function returns a plain `XID` instead of a `Region` to avoid adding Xfixes as a package dependency.