Remove bogus check
Fix SSAO perf

I was using wrong mipmap count for linear depth rtt.
Spped up SSAO

Avoid using cos/sin too much, use sum of cos/sin formula instead.
Speed up SSAO again

Use log2(xy) = log2(x) + log2(y), and approximate log2(alpha) =
log2(previous alpha) + .5.
Log2 is indeed a costly instruction.
Merge branch 'master' of https://github.com/supertuxkart/stk-code into ScriptEngine
Scripts can now create action triggers by themselves. Example in tutorial