Add better shadow

[zakorgy]

The current shadow implementation uses shadow mapping (render the screen from the lights view to a deth buffer, and use that depth to calucalte shadow), but is a bit hacky in the shader.
There are many more ways it can be improved:

• Adding multiple light sources
• Replace point light with directional light, using orthographic projection.
  See: http://www.opengl-tutorial.org/hu/intermediate-tutorials/tutorial-16-shadow-mapping/
  and https://learnopengl.com/Advanced-Lighting/Shadows/Shadow-Mapping
• Consider other shadow techniques than shadow mapping
• Experience with sampler2D instead of sampler2DShadow in the shader

[zakorgy]

The global light with orthographic projection for light and shadow was added in #17
Also sampler2DShadow was replaced with sampler2D in that PR, because it produces a better result