Screenshots look brighter than they are - saving alpha channel?

[satoshinm]

Taking with F2:

and taken with Shift-Cmd-4, Space, Enter:

The first screenshot taken by hitting the F2 shortcut is brighter than expected, especially noticeable around the text. First noticed with #175 (comment) - maybe related to alpha channels? Screenshots were added in #119 #127

[satoshinm]

Screenshots taken with F2 in the native app appear fine, so may be specific to WebGL:

On the web version, all I do is:

#ifdef __EMSCRIPTEN__
    EM_ASM(
        var url = document.getElementsByTagName("canvas")[0].toDataURL();
        var a = document.createElement("a");
        var timestamp = new Date().toISOString();
        a.setAttribute("download", "screenshot-netcraft-" + timestamp + ".png");
        a.setAttribute("href", url);
        a.click();
    );  
#else

whereas on native, glReadPixels(GL_RGBA) then compress to png with miniz tdefl_write_image_to_png_file_in_memory_ex. Something about premultiplied alpha? According to https://bugzilla.mozilla.org/show_bug.cgi?id=650720

It's also worth noting, that calling toDataUrl on the canvas causes it to be premultiplied before it gets to the encoder. The PNG decoder will un-premultiply it again.