Revert "make clearcolor black, so that space is dark instead of gray."

Revert "make clearcolor black, so that space is dark instead of gray."
This reverts commit b36b33f716031ef5933d41a1e5c17c6be3e54c28.

See #316: Caused issues with sky becoming black in dense fog.
Also: when the fog issue is solved, we could configure the constant
(black) clear color once only (during init), no need to do it in every
loop. Also, entire "if (skyblend) { ... } else {..}" was redundant,
since "if" and "else" clauses were all identical - and the "clearColor"
variables were unused. => Once the fog issue is solved, we can easily
improve the clearColor configuration here.