Add some code for performance test with different types of picture loading.
Pictures are not committed to Git because they are a little big.
fix a bug about vsnprintf. Note in Visual C++ and GCC the vsnprintf are different. see the comment in source code.
Add async resource loader for SDL_image, and libktx.

There are some multi-thread issue and I spend some time for debug.
It looks OK for now, but I'll keep this in mind for several days.

When the function 'main' returns, global/static objects seem not to be destructed,
I destroy them in 'main' function by hand.
Add async resource loader for SDL_image, and libktx.

There are some multi-thread issue and I spend some time for debug.
It looks OK for now, but I'll keep this in mind for several days.

When the function 'main' returns, global/static objects seem not to be destructed,
I destroy them in 'main' function by hand.