#41 Proper alpha compositing

[Anton Malov]

This patch fixes alpha compositing in ilBlit(...). There are two major changes:
1) Alpha compositing is now performed according to this document http://www.cs.princeton.edu/courses/archive/fall00/cs426/papers/smith95a.pdf so background and foreground alpha values takes part in computation.
2) Color rounding problem fixed by adding 0.5 to intermediate float value before truncation.

Howewer, alpha blending still works only for 4 bit per channel rgba due to (ILubyte) truncation.

[Anton Malov]

ilBlit alpha compositong patch