fixed text referencing GLFW library in SDL examples

Examples: Most examples apps detect if ImDrawIdx was changed to 32-bits instead of 16

Examples: OpenGL2/3: Save/setup/restore viewport as part of render function. Update SDL example to match GL2 one. (fix #392)

Examples: Possibly clarified the intent of imgui_impl_* files.

Examples: comment on setting the RenderDrawListsFn function.

Minor Windows adjustments

Use ImGuiIO::AddInputCharactersUTF8() in SDL2 example ...

Now Unicode text input works in this example.

Fix SDL2-example for Linux ...

The header is called "SDL_opengl.h" - case matters on Linux.

The main() function must indeed be called main(), on Windows it'll be
renamed to SDL_main (for SDL2main.lib) by #define in SDL_main.h
(which is included by SDL.h)

I added an entry to the example's README that describes how to build
it on Linux.

Merge remote-tracking branch 'origin' into 2015-03-antialiased-primitives

SDL example: tweaks (#233 #226)

SDL example: update for indexed rendering.

SDL example: more cleanup to match other examples (#233 #226)

SDL example: further cleanup to match other examples (#233 #226)

SDL example: moved event loop to main.cpp , adding page up/page down. (#226)

SDL example: renamed folder, added in README