Draw a line from (x, y) to (x1, y1) with color.
Source
Future<bool> line(int x0, int y0, int x1, int y1, int color) => proto.write(
6,
_s2b(x0)
..addAll(_s2b(y0))
..addAll(_s2b(x1))
..addAll(_s2b(y1))
..addAll(_s2b(color)));