Discard transparent pixels in fragment shader
Add 4-tap sampling to fix artifacts near texture boundaries

As described by @mikolalysenko in http://0fps.wordpress.com/2013/07/09/texture-atlases-wrapping-and-mip-mapping/

Before: http://imgur.com/O7kVUef,TSRZBwq#1
After: http://imgur.com/O7kVUef,TSRZBwq#0

Squashed commit of the following:

commit 2ae2a45a9d5a65ac65c54b98903139584107e65e
Author: deathcap <deathcap@gmx.co.uk>
Date:   Sun Jan 12 13:56:17 2014 -0800

    Add option useFourTap to enable/disable 4-tap sampling technique

commit 6c532231d3ccd1fecf2c21469bec0ae689e76313
Author: deathcap <deathcap@gmx.co.uk>
Date:   Sun Jan 12 13:42:46 2014 -0800

    Add 2x2 repeated images for 4-tap algorithm; fixes seams

commit 4dc2cbd6bad6dd370200ccfcfc981b7340ea1583
Author: deathcap <deathcap@gmx.co.uk>
Date:   Sun Jan 12 06:52:13 2014 -0800

    Attempt to use fourTapSample(), but I get seams.. down the middle