easings.h refinement and example

Add rewind for the XM music format

Fix unitialized struct member bug (#848)

Allow to reconfigure stdin to read input keys, this process could lead to undesired effects. Use with care.

Disabled by default.

Added comments for the future

Load glTF

When using FormatText() several times in same function, returned string is static and so, the same is returned, resulting in failures on shader loading.

From the Clang documentation[1]:
> -x<language>, --language <arg>, --language=<arg>
> Treat subsequent input files as having type <language>

Follow the advice. Fixes #840.
[1]: https://clang.llvm.org/docs/ClangCommandLineReference.html

This reverts #841 commit 97160fd970.

[build] fixed GLFW compiler flag for OSX

Contributed by ProfJski

Now shader uses `fragTexCoord` that are the full screen texture coordinates normalized, instead of `gl_fragCoord`, the unnormalized screen coordinates