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.

mods/default/README.txt

@@ -156,6 +156,9 @@ sofar (CC BY-SA 3.0):
   default_aspen_tree_top, derived from default_pine_tree_top (by paramat)
   default_aspen_wood, derived from default_pine_wood (by paramat)
 
+sofar (WTFPL):
+  default_gravel.png -- Derived from Gambit's PixelBOX texture pack light gravel
+
 Neuromancer (CC BY-SA 2.0):
   default_cobble.png, based on texture by Brane praefect
   default_mossycobble.png, based on texture by Brane praefect