improve demo code

[chirsz-ever]

• use Makefile as build system
• add .gitignore
• split demo code to separate files
• split platform code and render
• refactor atlas.inl as a separate font
• make the demo window resizable
• delete atlas texture when opengl shutdown
• add glfw3 implemention

[rxi]

I appreciate that some time and effort went into this pull request, but I don't consider this an overall improvement to the existing demo code. The demo is meant to be as terse and simple as possible -- a minimal example of how to get started using the library. Contrary to actual software it isn't meant to check every error case, it isn't meant to be nice or robust, it's meant to be as little "other code" to "code showing how the library works" as possible -- it's not an example on how to properly document your code, handle errors, manage resources or best use OpenGL, it's an example of how to set up and use the library, any focus elsewhere is just noise.

[0xtrzy]

how generating inl file?