[Renderers/Allegro4] Add Allegro 4.x renderer and demo

[eigenlenk]

Allegro 4 renderer

API conformance

• Rectangle (+rounded)
• Text
• Border (+rounded)
• Scissor
• Custom (accepts a function pointer to allow custom drawing within element)

Missing or incomplete features

• Image (missing resizing support and corner rounding)

Allegro 4 demo

Demo is rather random/silly and based on this screenshot of Microsoft Encarta 95:

Remade in Clay:

Same demo compiled for MS-DOS:

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So yeah, perhaps not the most useful renderer and demo since 4.x is rather old. But now that I have a better grasp of the library, I could try supporting the more modern 5.x version which is still actively developed. I also couldn't work out fetching and compiling the library through Make, so a pre-built binary is included instead 🤷‍♂️

[nicbarker]

Hello, sorry for the delay in getting to this. It looks so awesome 😁
Don't worry about not including the .dll, I'm sure people who are motivated to use the Allegro renderer will be able to source a copy of the lib themselves 🙂
Thank you for the hard work!

[eigenlenk]

I think I need to drop Make support for this and include a README instead. It's not going to build on Linux out of the box anyway, and only works with MinGW on Windows as well. Should silence these failing checks.