Improve Glass internal reflection

Fixes issue: http://www.yafaray.org/node/569

Increasing the internal limit for glass internal reflection gives more realistic detail in glass. As discussed with DarkTide, it will cause a small performance penalty, but the added realism is worth it and the performance penalty is not so big.
New params for Background Transparency
Merge pull request #53 from DavidBluecame/alpha_background

New params for Background Transparency
Merge pull request #51 from DavidBluecame/glass_reflection

Improve Glass internal reflection