Subpaths that become empty after rounding are not dropped

[rsheeter]

emoji_u1f6d2.svg from Noto Emoji has a path that changes if you run picosvg repeatedly. Track this down and fix it.

<!-- this path -->
  <path style="fill:none;stroke:#84B0C1;stroke-width:4.913;stroke-miterlimit:10;" d="M80.11,78.64l-69.94-4.66
    c-0.23-0.02-0.4-0.2-0.4-0.43V34.5c0-0.37,0.29-0.68,0.66-0.7l88.63-5.64c0.86-0.06,1.48,0.8,1.17,1.6L81.41,77.82
    C81.2,78.35,80.68,78.68,80.11,78.64z"/>

<!-- has the following subpath after stroke is applied  -->
M10.625162124633789,36.24893569946289 L10.562593460083008,36.2529182434082 Q10.594123840332031,36.25121307373047 10.625162124633789,36.24893569946289 Z

<!--
Unrounded the subpath above has a non-zero area (6.29364512860775e-06) and is retained. 
After rounding it has a zero area and will be dropped if remove empty subpaths runs again,
such as in a second round of picosvg'ing -->

[rsheeter]

Fixed by #270