[css3-shapes] Applicability of shapes

The current section 3.4.1. of shapes specifies that they apply to floats only. This is too restrictive and it assumes that implementations of the spec aren't implementing exclusions. We should revert this so that shapes apply to any block-level-block. This will allow implementations of shapes only to have the effect on floats. For implementations that also support exclusions the effect will apply to both floats and exclusions (block-level-blocks).

Thanks,
Rossen