[css-shapes] shape-image-threshold should be clearer about >= vs >

http://dev.w3.org/csswg/css-shapes/#shape-image-threshold-property
says:
  # <alphavalue>
  #   A <number> value used to set the threshold used for extracting
  #   a shape from an image. Any values outside the range 0.0 (fully
  #   transparent) to 1.0 (fully opaque) will be clamped to this
  #   range. 

which isn't clear about whether you're looking for pixels that are
greater than ( > ) the alphavalue or greater than or equal ( >= ) to
the alphavalue.

There is an example earlier:
  # A value of 0.5 means that the shape will enclose all the pixels
  # that are more than 50% opaque.
which suggests that it's greater than ( > ).

I tend to think it's actually preferable for it to be a >=, because
then 0.0 is useful as a default value that has the current behavior
(no image shape threshold) since all image pixels have an opacity
that are >= 0.

I think the alternative ( > ) doesn't seem as useful because:
 * using >, the 1.0 value doesn't seem useful (no shape, always)
 * the Initial value doesn't describe the current behavior, so
   another value would be needed

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)