Texture draw fix

[matyasatfp]

Textures have small visual artifacts around edges becasue of a rounding error, this commit fixes it

[tconkling]

I squashed these two commits together and pushed the resulting single commit to master: https://github.com/threerings/flump/commit/a94a386c2f33b39ec6a9a94b64549d3820ed8efe