Fix aalines overlap

[mzivic7]

As seen on the picture bellow, there is a problem with draw.aalines.

This is happening because draw.aalines require each aaline to be drawn 1px shorter (in some cases 2px - see bellow), so two connected draw_aaline are not overlapping.

What is changed and how it works:

draw_aaline has 2 endpoints that needs to be disabled for draw.aalines only:
First endpoint is disabled when it is first line when aalines is open.
And when from_x value is not decimal (if it is, then line is 2px shorter).
Second endpoint is disabled when it is last line when aalines is open.

As a result of disabling first endpoint, now there is special case where 1px is missing between two lines, where one is steep and other line is not.
This is detected in draw.aalines.
And resolved in draw_aaline by drawing one px at place where end point should be.
At what end point will be drawn pixel: first point when line is not inverted, second point when line is inverted.

Here are some results (might need to zoom to see the difference better):

Top line is modified.
Middle line is original.
Bottom line is difference.

Code used to draw this:

import pygame
import numpy as np

def curve_points(a, b, pea, t):
    sin_p = np.sin(pea)
    cos_p = np.cos(pea)
    x = a * np.cos(t)
    y = b * np.sin(t)
    x_rot = x * cos_p - y * sin_p
    y_rot = y * cos_p + x * sin_p
    return np.stack((x_rot, y_rot), axis=1)

points1 = [[200.1, 300],
           [220.1, 300.1],
           [240.1, 300.2],
           [260.1, 300.4],
           [280.1, 300.6],
           [300.1, 300.8]]

points2 = [[400.7, 300.1],
           [420.3, 300.5],
           [440.6, 305.1],
           [460.4, 315.2],
           [480.2, 330.5],
           [500.1, 360.7],
           [550.4, 360.5],
           [550.8, 400.2]]

points_num = 150
t = np.linspace(-np.pi, np.pi, points_num)
points3 = curve_points(250, 150, 0.3, t) + np.array([350, 350])

pygame.init()
screen = pygame.display.set_mode((700, 700))
clock = pygame.time.Clock()

run = True
while run:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
    screen.fill("black")
    pygame.draw.aalines(screen, (150, 150, 150), False, points1)
    pygame.draw.aalines(screen, (150, 150, 150), False, points2)
    pygame.draw.aalines(screen, (150, 150, 150), False, points3)
    pygame.display.flip()
    clock.tick(60)
pygame.quit()

Results were merged for comparison in gimp.

[oddbookworm]

I know this is asking for something that won't be fun, but I'd like to see some regression test(s) put in place that demonstrate this change, and will flag if the change gets broken by a future commit

[oddbookworm]

One small nitpick (that applies to both tests), but otherwise this looks good to me!

[oddbookworm]

This still looks good to me

[bilhox]

So I tried to understand how the algorithm works, but even after a lot of doliprane, I couldn't understand the algorithm because of lack of graphical explanations. I believe it would be better if @mzivic7 or anyone who understood the algorithm, make a video that explains step by step the algorithm for the sake of others mental health. Like that we can push this PR faster to its merge.

[MyreMylar]

OK, LGTM 👍

Output looks a lot better (Once I zoomed in, my old eyes couldn't see shit at normal resolution) and the changes make sense to me to reduce the overlapping in the original algorithm.