`background()` in `setup()` does not persist

In Processing (Java), calling background() inside setup() sets the initial canvas background, which remains visible unless explicitly changed in draw().

In libprocessing / mewnala, calling background() inside setup() appears to have no effect once draw() starts.

Example

from mewnala import *

def setup():
    size(600, 600)
    background(255)

def draw():
    fill(255, 0, 0)
    rect(100, 100, 200, 200)

run()

See screenshot below for output.

Expected behavior (Processing Java)

White background with a red rectangle drawn on top.

Actual behavior (libprocessing / mewnala)

Black background with a red rectangle, suggesting the background set in setup() is cleared or ignored when draw() runs.

Screenshot

先使用報告中的 setup()、draw() 和 run() 範例重現問題，然後追蹤 setup() 與 draw() 之間的算繪生命週期。確認實作後的行為會讓 setup() 中的白色背景保持顯示在紅色矩形下方，與 Processing Java 一致。