Updated screen blend to account for alpha in both textures

[giilby]

This is a fix for issue #1268.

[BradLarson]

Does this anticipate premultiplied alpha in the input images, or is it for the case where there isn't premultiplied alpha in the inputs? I'm trying to eliminate premultiplied alpha from my inputs, so I wanted to check.

[giilby]

The updated shader does not expect premultiplied alpha in the inputs, as it performs the premultiplication itself, then "undoes" it before setting gl_FragColor

[giilby]

Brad, is there anything preventing these changes from being pulled into master? Let me know if there's something I need to fix!

[kakashilw]

Hello, how do we pass the alpha parameter?

[ygsky]

来信已收到，我会尽快查看！