Fix memory usage

[fenollp]

Right now a very static vertex buffer is allocated on host memory, this can probably be done directly on the GPU with geometry shaders (see layout). By default this buffer is full and of size 256 * 256 * 256 * 3 * 4, or about 200MB.

Also, it is of note that the colour buffer is mostly empty most of the time, while it is of the same size as the vertex buffer.
To fix this I am thinking of first processing the input file, turning it into a texture, then moving the sliding window on this texture.