Changed background color for new Layers to black insted of white

[HiCamino]

the app seems to expect to be black bg for everything else

(even color selector shows bg as 0, 0, 0 beFORE this fix!!!)

Fixes #684

[humdingerb]

I'm not sure I 100% understand what's described in #684...
But I assume you know what you're doing and this change makes the internals more coherent. :)

[HiCamino]

easiest way to understand it is if you open a blank image and use the color selector you get 0, 0, 0, 0 which is black 100% alpha

then use the transparency tool on the blank image and you find out the image is actually 255, 255, 255, 0 aka WHITE 100% alpha

the transparency tool only changes transparency and not color so the blank image really is white.
so the color selector is lying to you

but also other tools think black is "clear" like if you use the Eraser on the blank image and then use transparency tool on the spot you just erased it is black

and also if yoiu open an image with a transparent bg it is aslo black:

i hope this makes sense the only time a transparent image is white is when you make a new layer otherwise everything else says "clear" = black 100% alpha

[HiCamino]

added hopefuly easier explanatoin in #684

[humdingerb]

I think I get it... at least the behaviour when using the eraser and transparency tools is now correct after this PR got merged.
However, the colour picker didn't lie: the current ArtPaint 2.7.2 from HaikuDepot has the colour picker say it's 255,255,255,0 when used on a transparent canvas. Anyway... this was a good change. :)