Include guards in most changed files were missing, I added them keeping
the same style as other SDL files. In some cases I moved the include
guards around to be the first thing the header has to take advantage of
any possible improvements compiler may have for inclusion guards.
Vitaly Novichkov
Once I ran build of my codecs collection on AppVeyor where my CMake script downloads latest SDL2 from HG repo, failed to link because of math functions conflict:
https://ci.appveyor.com/project/Wohlstand/audiocodecs/build/1.0.44
The revision is b9ff5f8b2303
There are both vanilla MinGW and MinGW-w64 are failed to build.
```
[100%] Linking C shared library libSDL2.dll
c:/mingw/bin/../lib/gcc/mingw32/5.3.0/../../../libmingwex.a(scalbn.o):(.text+0x0): multiple definition of `scalbln'
CMakeFiles\SDL2.dir/objects.a(s_scalbn.c.obj):C:/projects/audiocodecs/build-MinGW-Release-Win32/external/SDL2/src/SDL2HG/src/libm/s_scalbn.c:30: first defined here
The internal function SDL_EGL_LoadLibrary() did not delete and remove a mostly
uninitialized data structure if loading the library first failed. A later try to
use EGL then skipped initialization and assumed it was previously successful
because the data structure now already existed. This led to at least one crash
in the internal function SDL_EGL_ChooseConfig() because a NULL pointer was
dereferenced to make a call to eglBindAPI().
