Simplified logic flow of box3d

Some if-branching was either unnecessarily complicated, redudant or hard to read.

Nested if statements with no else-part were reduced into one statement with all expressions connected by '&&'.

Varaibles which were declared outside of a branch but only needed within it are now declared in the branch/statement.

Some variables were declared, not used and then initalized later, this is now put together.

Finally there still was C-style casting, which is now updated to Cpp style casting.