Add Microsoft Visual C++ project for SDL_image
solve a minor warning message given by GCC
minor fix for Microsoft Visual C++ project
modify main.cpp to test loading a WebP picture, and display it using OpenGL(ES).

On Android mobile phones it works fine. (armeabi, armeabi-v7a are both fine. That is, with/without NEON, both fine).
But on windows it seems not working good as I expected.
I've tested several pictures (with/without SSE, with/without alpha channel), all these picture are looking very green (many pixel are decoded as RGB(0, 255, 0)).
Think will fix this, or I cannot use WebP as my primary file format for the moment.
modify the Visual C++ project file, add /DHAVE_LIBC simply solve the "green WebP picture" bug.

I haven't find out why, but it works.
Note that in Visual C++ I didn't use the official SDL makefile, but create a SDL project by myself.
So it is not a surprise to find some strange bugs in Windows version.

In my test, the official SDL.lib (SDL 2.0.3, downloaded directly from http://www.libsdl.org) does work fine, too.
On Android platform I use the official Android.mk with only a little modification. Hope it works fine as well.
upgrade libwebp from 0.3.0 to 0.4.0 (manual merge, take care ...)
these folders "mux", "demux" are not upgraded. since I am not going to use them for the moment.
Found the real reason of the "green WebP picture" bug, and solve it.
I file this bug at https://bugzilla.libsdl.org/show_bug.cgi?id=2537
Add some code for performance test with different types of picture loading.
Pictures are not committed to Git because they are a little big.