The alphamask example: The web preview doesn't match the Processing example

Issue description

The alphamask example https://processing.org/examples/alphamask.html
has a P5.js preview over the code. When the Processing example is loaded
from the Processing editor - the sketch doesn't behave the same way as
the preview.

My impression is that the P5js preview on top of the code needs to invert
the alphamask in order to behave the same way as the Processing example below.

See screenshots of the sitatuation here:

https://github.com/processing/p5.js/issues/5343

URL(s) of affected page(s)

https://processing.org/examples/alphamask.html

Proposed fix

Invert alphamask of the image used for the P5.js preview (and do nothing
for the image used as alphamask in the Processing examples).

从 processing.org/examples/alphamask.html 的 alphamask 示例页面开始，检查其 P5.js 预览如何使用 alphamask 图像。将预览与下面的 Processing 示例进行比较；当两者行为一致，且预览应用所提议的反转而 Processing 示例保持不变时，即表示完成。