New gravel texture.

New gravel texture.

Issue #811 - new gravel texture needed.

This texture was Gambits' PixelBOX gravel light texture. Gambit
posted that his texture pack is WTFPL:
- https://forum.minetest.net/viewtopic.php?f=4&t=4990&start=50#p141196

I've made significant modifications to this texture:
- slightly rotated and rolled some sections of pixels
- minor burn/dodge some pixels to keep high contrast
- removed lineair repeating effects
- etc.

Attribution is added back to Gambit. Thanks.