[fb] Remove extraneous m_lines from scrollback

We have one big array of characters in scrollback, with lines of
constant width -- there's no reason to have an array of pointers when
offsets will do.

src/drivers/fb/scrollback.cpp

@@ -12,10 +12,6 @@ scrollback::scrollback(unsigned lines, unsigned cols, unsigned margin) :
 	m_margin {margin}
 {
 	m_data = reinterpret_cast<char*>(malloc(lines*cols));
-	m_lines = reinterpret_cast<char**>(malloc(lines*sizeof(char*)));
-	for (unsigned i = 0; i < lines; ++i)
-		m_lines[i] = &m_data[i*cols];
-
 	memset(m_data, ' ', lines*cols);
 }
 
@@ -27,15 +23,17 @@ scrollback::add_line(const char *line, size_t len)
 	if (len > m_cols)
 		len = m_cols;
 
-	memcpy(m_lines[i], line, len);
+	char *start = m_data + (i * m_cols);
+	memcpy(start, line, len);
 	if (len < m_cols)
-		memset(m_lines[i]+len, ' ', m_cols - len);
+		memset(start + len, ' ', m_cols - len);
 }
 
 char *
 scrollback::get_line(unsigned i)
 {
-	return m_lines[(i+m_count)%m_rows];
+	unsigned line = (i + m_count) % m_rows;
+	return &m_data[line*m_cols];
 }
 
 void
@@ -48,7 +46,7 @@ scrollback::render(screen &scr, font &fnt)
 	const unsigned ystride = (m_margin + fnt.height());
 
 	for (unsigned y = 0; y < m_rows; ++y) {
-		char *line = get_line(y);
+		char *line = &m_data[y*m_cols];
 		for (unsigned x = 0; x < m_cols; ++x) {
 			fnt.draw_glyph(scr, line[x], fg, bg, m_margin+x*xstride, m_margin+y*ystride);
 		}

src/drivers/fb/scrollback.h

@@ -17,7 +17,6 @@ public:
 
 private:
 	char *m_data;
-	char **m_lines;
 	unsigned m_rows, m_cols;
 	unsigned m_start;
 	unsigned m_count;