Implements SDL backend with working example using OpenGL 3

[nadous]

Everything this PR does is in the title with a couple of side notes:

• I'm working on Linux with my Zig compiler being set on master
• I took the liberty of cleaning up build.zig & moving shaders to a different location
• SDL dependency comes from SDL.zig from where I've only copied the required files, I could refactor my fork so it's a submodule.
• I plan on implementing vulkan

[SpexGuy]

Thanks for submitting this! I'm happy to merge it but it needs a few changes first:

• the SDL.zig/ folder should be moved into the examples/include/ directory
• please restore the trailing newline in .gitignore
• I should still be able to build the non-SDL examples without installing SDL. I get this message instead when building example_glfw_vulkan:

> zig build example_glfw_vulkan
Could not auto-detect SDL2 sdk configuration. Please provide C:\Users\Martin\Projects\Zig-ImGui\.build_config\sdl.json with the following contents filled out:
{
  "x86_64-windows-gnu": {
    "include": "<path to sdl2 sdk>/include",
    "libs": "<path to sdl2 sdk>/lib",
    "bin": "<path to sdl2 sdk>/bin"
  }
}

You can obtain a SDL2 sdk for windows from https://www.libsdl.org/download-2.0.php
error: the following build command failed with exit code 1:
C:\Users\Martin\Projects\Zig-ImGui\zig-cache\o\83cb1bbbb8bded64fa9d40dbd38b4b9f\build.exe C:\Program Files\Zig\Current\zig.exe C:\Users\Martin\Projects\Zig-ImGui C:\Users\Martin\Projects\Zig-ImGui\zig-cache C:\Users\Martin\AppData\Local\zig example_glfw_vulkan