PNG with color type grayscaleAlpha has wrong behaviour

[NicolaVerbeeck]

1. It decodes as having no alpha (given that alpha is defined as numChannels == 4) this would fail (numChannels == 2)
2. Converting it to a 4 channel image causes a wrong conversion

Test data:

Resulting converted image:

[brendan-duncan]

I pushed an update for this in 9f39c25

[NicolaVerbeeck]

You sir, are awesome!

Will take a look at it

[NicolaVerbeeck]

Checked it and now it works though we can't determine if the original image has alpha. Any ideas?

[brendan-duncan]

2 channel images are almost always luminance-alpha. I see Image.hasAlpha wasn't updated, I'll do that. Pixel r, g,, b, and a methods were updated.