Issue #25 -- Vertical edges of boxes run together

Regression Testing
---
Ran ./pre-push: SVG output of three files changed.  Log excerpt:

    === RUN   TestExamples
        ...
        examples-regression_test.go:83: Failed to verify contents of 3 .svg files
        examples-regression_test.go:130: Writing new SVG and HTML delta files into _examples_new/
        examples-regression_test.go:149: 	complicated.html
        examples-regression_test.go:149: 	regression.html
        examples-regression_test.go:149: 	small-grids.html
        ...

Visual inspection of .html files in browser shows fix effective in
_examples_new/small-grids.{html,svg}, and no visually apparent change
in the other two sets of results.

drawable.go

@@ -441,6 +441,10 @@ func (c *Canvas) getLines(
 			currentLine = snip(currentLine)
 		}
 
+		if o == S && (r == '.' || lastSeenRune == '\'') {
+			currentLine = snip(currentLine)
+		}
+
 		switch currentLine.state {
 		case _Unstarted:
 			if shouldKeep {