lavapipe: dummy external exts
The external memory/fd/semaphore extensions are needed for 1.1 in the future, but we don't have to support the underlying OS extensions, so just add the simple dummy support for these that is sufficient for CTS to pass.
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The external memory/fd/semaphore extensions are needed for 1.1 in the future, but we don't have to support the underlying OS extensions, so just add the simple dummy support for these that is sufficient for CTS to pass.