Fix compile warnings in SDL examples

Examples: SDL+OpenGL: explicitly setting GL_UNPACK_ROW_LENGTH to reduce issues because SDL changes it (#752)

Examples: SDL: Fixed unused variable warning on non-Windows platforms (#626)

Fixed various Clang -Weverything warnings (#626)

IO: Added "Super" keyboard modifiers (corresponding to Cmd on Mac and Windows key in theory although the later is hard to read) (#473) ...

NB: Value not used.

Trim trailing spaces

Examples: SDL: Made ImGui_ImplSdlGL3_NewFrame() signature match GL2 one

Examples: SDL: Minor fixes to follow syntax of other examples (#495)

Applying same fix to OpenGL 3 example. Fixing spaces/tabs.

Automatically updating DisplayFrameBufferScale by reading the OpenGL drawable size and comparing with the window size. This fixed dear imgui which was rendering only to 1/4 of my window.

FAQ and comments about the use of ImTextureID (#562, #561, #521, #510, #497, #475 ...)

Examples: OpenGL: Fix early return on zero-sized framebuffer breaking GL state (#486, #547)

Examples: OpenGL: skip rendering and calling glViewport() if we have a zero-fixed buffer (#486)

Examples: SDL/OpenGL: Tabs to spaces

Examples: OpenGL: Tentative fix for miscaled call to glViewport on high-DPI screens (#441) ...

Fix for 980ca28

Examples: Not clearing input data/tex data in atlas (will be required for dynamic atlas anyway). Effectively fix resizing in DX examples. ...

+ Standardized comments.

Examples: Header comments.

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)