Consider replacement of GraphicsMagick

[ArmorDarks]

We should consider replacement of GraphicsMagick with Jimp.

So far only two our tasks dependent on it:

• grunt-spritesmith — dependency can be avoided by using pixelsmith engine
• grunt-responsive-images — potentially can be polyfilled with grunt-jimp, but we should check how well it works with tricky images, like transparent pngs