Example and GLEW cleanup

Simplify example code and make GLEW completely optional. Next steps will be to
remove the SDL dependency from the core three library, and continue moving
header code into implementation sources.
Cleanup of glext.h usage on Windows.

Non-GLEW builds aren't really functional on Windows, but we're getting there.
Move example-specific code into examples/.

There's no need to litter the main library with examples-specific helper code.
Massive SDL, GLEW and GL update

Remove GLEW dependency entirely.
Remove GL linkage dependency from the core library.
Switch to SDL2.
Simplify example interface.
Fix issue with HSL color calculation.
Change example include style
Remove glew entirely

Remove glew and add appropriate SDL2 libraries for Windows. Also fix several
compilation issues with MSVC.
Support for GLES compilation on Android.