Make the default:grass texture tile better

It bothered me every time I saw the grass texture, so I ran it through Gimp's Resynthesizer plugin and came up with this.

Rather than sit on my own changes to scratch my own itch, I figured I'd share. :)

## Before
![default_grass_original_tiled](https://f.cloud.github.com/assets/346877/692310/97ef037e-dc18-11e2-9e1f-1bc74a2ad630.png)
![screenshot_1906448945](https://f.cloud.github.com/assets/346877/692318/97cef84e-dc19-11e2-9994-838b28e19dd9.png)

## After
![default_grass_tiled](https://f.cloud.github.com/assets/346877/692305/33f7dc38-dc18-11e2-80d7-1e40dfc22d41.png)
![screenshot_1906596236](https://f.cloud.github.com/assets/346877/692320/a3182a36-dc19-11e2-87c3-9b718e22a47e.png)