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).

processing.org/examples/alphamask.html の alphamask サンプルページから始め、P5.js プレビューで alphamask 画像がどのように使われているかを調べてください。プレビューを以下の Processing サンプルと比較してください。プレビューが提案された反転を適用し、Processing サンプルは変更されない状態で、両方が一貫して動作すれば完了です。