Add "connected" glass and obsidian glass

This gives glass and obsidian glass a slight 3D look and make them look “connected” to other nearby nodes of the same type.

Compatibility with older texture packs is kept. Dropped glass looks better and is now 3D.

**default_(obsidian_)glass.png**: inventory and wield texture, compatible with old texture packs
**default_(obsidian_)glass_frame.png**: the glass frame
**default_(obsidian_)glass_detail.png**: the glass detail

![Glass](https://cdn.mediacru.sh/aVki53_m3IgG.jpg)
I opened a pull request for removing `inventory_image` from the nodedef, but it caused problems with the inventory image. I tested it just again, and these are the results:

Without `inventory_image`, shaders enabled:
![2014-09-27 - 17 47 10 capture](https://cloud.githubusercontent.com/assets/2388420/4432148/21645b42-4688-11e4-9c73-bd6b6669d5b4.png)

Without `inventory_image`, shaders disabled:
![clipboard02](https://cloud.githubusercontent.com/assets/2388420/4432153/4ec9a876-4688-11e4-88c5-284849ccae16.png)

With `inventory_image`, shaders disabled:
![clipboard03](https://cloud.githubusercontent.com/assets/2388420/4432152/4aac708e-4688-11e4-82de-3756fa651b63.png)