Modernized ray-tracing

[kvark]

Algorithm is based on Maximum Mipmaps for Fast, Accurate, and Scalable Dynamic Height Field Rendering.

• update glutin and configuration
• generate the max mipmap chain

• actually rewrite the raytracing algorithm
• support dual layer
• make it portable

[kvark]

Positive direction rays are almost perfect now:

[kvark]

Debug mode matrix:

[kvark]

Dual layer is here 🎉 :

[kvark]

I think the change is basically done. Of course quality/performance improvements are possible here and there, but this will always be the case.

The biggest concern is currently the portability of the change:

• doesn't work on Mac/Intel (not sure about dedicated HW macs), since their GL doesn't support texture views
• doesn't work on Linux/Intel, because of apparently a driver bug causing the shader to not see any mipmaps of sampler2DArray texture:
• texelFetch with non-zero LOD always returns zero
• textureLod with non-zero LOD always returns the data from the base LOD

With this in mind, I think we should proceed with the change, considering the follow-ups:

1. another way to render the terrain that is way simpler than ray tracing or tessellation
2. eventually port to wgpu-rs, which would resolve both the bugs and limited GL capabilities

[kvark]

Alright, I did the most reasonable thing for portability - brought back the old ray tracer, just as a fallback option. Time to ship this!
bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push