fix the premultiply in shockwave filter

[GoodBoyDigital]

fixes #443

[bigtimebuddy]

This definitely fixed the alpha issue, but now brightness property is not being respected.

• https://jsfiddle.net/bigtimebuddy/amt9gnd7/ (webgl, expected)

  

• https://jsfiddle.net/bigtimebuddy/xthspjwu/ (webgpu, actual)