Add SDL2 + OpenGL 3 backend

[mjakeman]

Hello!

This adds an SDL2 backend based on the upstream IMGUI one, with the relevant zgui specific changes.

See also: zig-gamedev/zsdl#7 (adds necessary header files to zsdl)

Zig Impl changes:

• Adds new backend_sdl2 and backend_sdl2_opengl implementations modelled after the GLFW ones

IMGUI Backend:

• Taken from this commit from August: ocornut/imgui@2d99052
• Not using the most recent version as it has some API changes which this repo's IMGUI build doesn't have
• Minor changes to extern in line with the GLFW build, commented with // FIX(zig-gamedev) to match

Build changes:

• Migrates from lib.defineCMacro to lib.root_module.addCMacro as that's been removed in master

To Do:

• Needs a version of zsdl containing zig-gamedev/zsdl@9c87bd9

[mjakeman]

Somewhat addresses #5

[hazeycode]

Looks good, thanks !