[css-shapes] future use of shape-image-threshold for luminance

Hey all,

Based on some discussion yesterday and today, I've added the stub of a
section to Shapes Level 2 to define an alpha/luminance switch that would
control whether extracting a shape from an image would use luminance data
rather than alpha data. And I've somewhat sanitized the definition of
shape-image-threshold to allow its use with this switch to have the
threshold apply to either luminance or alpha values.

Thanks,

Alan