Fixed crash in TaskScheduler Windows implementation

Also replaced CreateEventA calls by CreateEventW calls to avoid one
level of function call.
Thanks to bacelar
Fixed typo
Replaced CreateSemaphore call by CreateSemaphoreW
Fixed warning on some compilers
Removed explicit VectorI::operator[]

Since Vector instances can be implicitely converted to T*, native
operator[] will work on them
Allowed asserts to throw exceptions instead of calling std::exit
Fixed possible bug
Fixed Image::GetPixels returning wrong pointer when level is over 0
Fixed build error in debug mode