Improve the SDL2 Surface RAWFB Demo

[BieHDC]

Hello Friends, i have made some improvements to the SDL2 Surface RAWFB Demo. Please review the changes and tell me other improvements i should/could make.

Makefile:

• Set c-std to c99

main.c:

• Remove translate_sdl_key and sdl_button_to_nk, replaced by nk_sdl_handle_event in sdl2surface_rawfb.h
• Add support for the further examples (style, calc, overview, node editor)
• The new main has been copied over from sdl2_opengl2 to match the other examples
• Added more errors checks for SDL2 returns
• Added support for resizing the window
• Other General Cleanups

sdl2surface_rawfb.h:

• Added nk_sdlsurface_update to support window resizing
• Moved nk_sdlsurface_init to a more correct place
• Only warn on NK_COMMAND_ARC/K_COMMAND_ARC_FILLED instead of asserting
• Add nk_sdl_handle_event (imported from sdl2_opengl2)
• Some indention fixes

Thanks to everyone else who has been working on this awesome project!

[BieHDC]

Ok, i downgraded it to a draft, because i spent some more time in it and there are a few more things to fix. Please stand by.

[BieHDC]

Fixed the merge conflict. Ready to merge.