Fix software rendering issues

I've been unable to use software mode in openfl due to several bugs:

- 8-bit PNGs with transparency are loaded/blitted as opaque
- Tile rasterization looks terrible

This pull request fixes both issues. The png alpha test just required an extra check and the poor tile rasterization was due to some naive (?) attempt at rounding. I'm a little unclear on what the +0.5 is supposed to do. Some of it looks like bad rounding when converting to fixed point (there are other incidents of this that I didn't change), and some of it looks like sampling from pixel centers.

The attached image demonstrates the rasterization bug and fix.

![limesoftwarefix](https://f.cloud.github.com/assets/5263143/1995245/14b8df7e-8501-11e3-8690-9f55989c4049.png)