Remove calls to Truncate() from linebreak parser

Replace output truncation with appropriate inline callbacks. lineBreak()
is now only responsible for handling HardLineBreak. BackslashLineBreak
is handled in escape() and trailing whitespace is considered in
maybeLineBreak().

inline.go

@@ -155,31 +155,28 @@ func codeSpan(p *parser, out *bytes.Buffer, data []byte, offset int) int {
 }
 
 // newline preceded by two spaces becomes <br>
-// newline without two spaces works when EXTENSION_HARD_LINE_BREAK is enabled
+func maybeLineBreak(p *parser, out *bytes.Buffer, data []byte, offset int) int {
+	origOffset := offset
+	for offset < len(data) && data[offset] == ' ' {
+		offset++
+	}
+	if offset < len(data) && data[offset] == '\n' {
+		if offset-origOffset >= 2 {
+			p.r.LineBreak(out)
+			return offset - origOffset + 1
+		}
+		return offset - origOffset
+	}
+	return 0
+}
+
+// newline without two spaces works when HardLineBreak is enabled
 func lineBreak(p *parser, out *bytes.Buffer, data []byte, offset int) int {
-	// remove trailing spaces from out
-	outBytes := out.Bytes()
-	end := len(outBytes)
-	eol := end
-	for eol > 0 && outBytes[eol-1] == ' ' {
-		eol--
+	if p.flags&HardLineBreak != 0 {
+		p.r.LineBreak(out)
+		return 1
 	}
-	out.Truncate(eol)
-
-	precededByTwoSpaces := offset >= 2 && data[offset-2] == ' ' && data[offset-1] == ' '
-	precededByBackslash := offset >= 1 && data[offset-1] == '\\' // see http://spec.commonmark.org/0.18/#example-527
-	precededByBackslash = precededByBackslash && p.flags&BackslashLineBreak != 0
-
-	// should there be a hard line break here?
-	if p.flags&HardLineBreak == 0 && !precededByTwoSpaces && !precededByBackslash {
-		return 0
-	}
-
-	if precededByBackslash && eol > 0 {
-		out.Truncate(eol - 1)
-	}
-	p.r.LineBreak(out)
-	return 1
+	return 0
 }
 
 type linkType int
@@ -615,6 +612,10 @@ func escape(p *parser, out *bytes.Buffer, data []byte, offset int) int {
 	data = data[offset:]
 
 	if len(data) > 1 {
+		if p.flags&BackslashLineBreak != 0 && data[1] == '\n' {
+			p.r.LineBreak(out)
+			return 2
+		}
 		if bytes.IndexByte(escapeChars, data[1]) < 0 {
 			return 0
 		}

markdown.go

@@ -376,6 +376,7 @@ func MarkdownOptions(input []byte, renderer Renderer, opts Options) []byte {
 	p.inlineCallback['&'] = entity
 	p.inlineCallback['!'] = maybeImage
 	p.inlineCallback['^'] = maybeInlineFootnote
+	p.inlineCallback[' '] = maybeLineBreak
 
 	if extensions&Autolink != 0 {
 		p.inlineCallback[':'] = autoLink