Corresponds to tk8.7a5

TkSVG derived from SVGNano support of Tk8.7a5.
This aims any version of Tk8.6.

A successful "package require tksvg" is also possible on 8.7 now and is a no-op.
Thanks to Brad Lanam and Jan Nijtmans for the fruitful discussions.

Changes:

1. Bug fix: possible number overflow:
   tk ticket: https://core.tcl-lang.org/tk/tktview?name=822330269b
   tksvg ticket: #14

2. Upstream fix from nanosvg
   #179: "Division by float 0"

3. pckIndex.tcl changed to load in tk8.7 as a no-op