OpenGl Backend

[VectorWolf]

Currently we use SDL2 for about every hardware interaction, but this comes with some limitations. With an own OpenGl backend we can:

• have pseudo 3d rendering via a shader which can take normals and light sources into account
• have fade in and fade out effects for single tiles or even whole layers
• have the possibility to use post processing shaders on layers for cool effects

[VectorWolf]

If I ever get to implement this, I should pay attention to only use the opengl es2/webgl subset of opengl to not break compatibility with emscripten (and future android builds)