Fixes some rendering related things.

* Fixes that the blur setting in the global options screen had not effect at all
* Fixes that the cube map of the sky dome was overly bright when running the game without postprocessing effects enabled
* Changes that water by default always reflects the skysphere and that the costly reflection of the world can be enabled via the options
* Renames "Reflective water" to "Water world reflection" respectively
* Finally fixes the broken ambient occlusion algorithm which caused that the left, right, back and front sides of blocks were occluded all the time
* Tweaks ambient occlusion intensity accordingly
* Removes the blueish tint of water blocks/planes
* Updates menu background images
Adjusts README.
Fixes and tweaks some rendering related things.

* Fixes various quirks in combination with reflective water
* Changes that water is more or less opaque when diving
* Removes some unnecessary normalize calls in the chunk fragment shader
* Replaces Fresnel term