USDLayerWriter fixes

[johnhaddon]

This fixes #6197, and also cleans up the contexts in which USDLayerWriter evaluates the input scenes.

[ericmehl]

I found a way to set a directory to be read-only on Windows - it's not pretty but at least it's a one-liner. chmod doesn't do the job for directories, and Windows' convoluted permissions scheme needs a little more prodding to get it to prevent writing into a directory.

If that seems like a good solution, it's probably worth doing the same for

gaffer/python/GafferImageTest/CatalogueTest.py

Lines 618 to 619 in 0b6cb39

	@unittest.skipIf( os.name == "nt", "Windows allows new files in read-only directories" )
	def testNonWritableDirectory( self ) :