Add more `copyExternalImageToTexture` tests as Chrome/Safari/Firefox are failing to agree how to load an image

[greggman]

this example: https://jsgist.org/?src=e9261816382d2637f29e8e7f105c1c40

gets

• Top-left RGBA: 255 255 255 1 on Firefox
• Top-left RGBA: 255 0 0 1 on Safari
• Top-left RGBA: 130 101 73 1 on Chrome

I think Chrome is correct (130, 101, 73, 1) is what's in the file.

This issue might be from createBitmapImage?

[greggman]

Trying with Image instead of createImageBitmap

https://jsgist.org/?src=2359aec968f72ec4b24f69c712753d59

• Chrome: Top-left RGBA: 130 101 73 1
• Safari: Top-left RGBA: 1 0 0 1
• Firefox: Top-left RGBA: 130 101 73 1