* FloatToInt() should now round correctly ot nearest, even for
negative numbers
* Log2() now calls log2f() instead of log10()*3.321 - no idea what the
previous code was about, I doubt it's faster (and the ASM code uses
something like log2, too).
* FastLog2() (for integers) now uses __builtin_clz() when building with
GCC/clang - the resulting ASM should be pretty similar to the inline
ASM below. I wonder why that function takes signed ints, log2(-1) in
reality is an irrational number (but the function returns 31)..
Also, both the inline ASM and my version return 0 for Log2(0), but
INT_MIN would be closer to the truth
* commented out FastMaxLog2(), it's unused.
* implemented _rotl() using a fast(er) trick from
http://blog.regehr.org/archives/1063
8d26863a51