Add first round of patches from assimp fast_atof version. This code is slightly slower than the original one, but for the first time works correctly according to the assumptions of the API docs. Especially stopping at the first non-supported character and returning properly MAX_INT or similar values. This obviously means that some more steps need to be done, but parsing shouldn't be the major performance bottleneck anyway.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3672 dfc29bdd-3216-0410-991c-e03cc46cb475