Account for fractions of a pixel when drawing

Previously, the endpoint of the line was always moved
along in increments of 1 pixel, so that the endpoint would always be
rounded down. This could accumulate to give quite large differences
from what the program intended.

Ensure that "goto" always ends up storing the floating point endpoints
and that the line is drawn from the rounded-integer starting coordinate
and rounded-integer ending coordinate.

This makes the 3 test lines in the OP's "turtle_truncate.txt" example
be the same length.

Closes: #41

Author: jepler

adafruit_turtle.py

@@ -407,13 +407,17 @@ def goto(
         xn: float = x1[0] if y1 is None else x1  # type: ignore
         xn += self._w // 2
         yn = self._h // 2 - yn
-        x0 = self._x
-        y0 = self._y
         if not self.isdown():
             self._x = xn  # woot, we just skip ahead
             self._y = yn
             self._drawturtle()
             return
+
+        self._do_draw_line(round(self._x), round(self._y), round(xn), round(yn))
+        self._x = xn
+        self._y = yn
+
+    def _do_draw_line(self, x0: int, y0: int, xn: int, yn: int):
         steep = abs(yn - y0) > abs(xn - x0)
         rev = False
         dx = xn - x0
@@ -444,15 +448,11 @@ def goto(
                     self._plot(int(y0), int(x0), self._pencolor)
                 except IndexError:
                     pass
-                self._x = y0
-                self._y = x0
             else:
                 try:
                     self._plot(int(x0), int(y0), self._pencolor)
                 except IndexError:
                     pass
-                self._x = x0
-                self._y = y0
             if self._speed > 0:
                 if step >= self._speed:
                     # mark the step