[Shader] Automatically generate black outlines

[Luis-Tavares]

The foreground tiles must have their outlines generated automatically by code. I'm currently using color #1b1b1b for outlines, but we can try other tones of black. It's a little hard to notice the difference right now, but this is very important for some brighter areas.

[ted537]

Steps to Implement

For each layer:

• (1) Render all tiles to a texture (instead of directly to the viewport)
• (2) Run rendered tiles through an edge detection filter / kernel to get border

The result of (2) can be rendered directly to the viewport.

https://en.wikipedia.org/wiki/Edge_detection