Fix Stained Glass pane rendering issues

This change to the Stained Glass Pane renderer fixes them not connecting to non-opaque blocks with solid side surfaces. For example, if a block has to return `false` in `isOpaqueCube` because there are non-opaque sub-blocks, but also has a solid sub-block that returns `true` on all `isSideSolid` and wants Glass Panes to connect to it, Glass Panes do not connect to the block.
**Before:**
![2014-08-18_20 18 10](https://cloud.githubusercontent.com/assets/4145923/3955885/cfaeb802-2705-11e4-96ef-149281d04fb8.png)
**After:**
![2014-08-18_20 19 37](https://cloud.githubusercontent.com/assets/4145923/3955886/cfb1add2-2705-11e4-9af0-819eebfafc7b.png)

However, this cannot be intended, since the hitbox correctly connects to the block.
**Before:**
![2014-08-18_20 21 11](https://cloud.githubusercontent.com/assets/4145923/3955887/cfc7ba1e-2705-11e4-9cfe-c785feca95be.png)
![2014-08-18_20 21 22](https://cloud.githubusercontent.com/assets/4145923/3955889/cfc8a4c4-2705-11e4-8383-f31dbd633124.png)
**After:**
![2014-08-18_20 20 02](https://cloud.githubusercontent.com/assets/4145923/3955888/cfc804e2-2705-11e4-817b-639322509d37.png)

This patch fixes that issue.