Add precise to tessellation fill_cover_* tests

- Add precise to misc_draw.fill_cover_* to avoid gaps in tessellated edges.
- Remove misleading comments.
- Add comments to describe why precise is not required to some tess eval
  shaders.

Bug: 19462784
Change-Id: Ia5e49c1b0cc0edd0e3c5d902e7b1f19dd841a9ac
3 files changed