Move common code according to case distinctions

[elfring]

Some function calls are repeated according to case distinctions. :thinking: Thus I imagine that a bit of common source code can be moved.

• Adjustments for switch statements
• Refactoring of if statements by using the conditional operator

[DTolm]

The tree structure that is implemented right now makes it easier to understand which shader is launched for a specific configuration (at least for me). The whole FFT Plan configurator can be changed at the later stage of development so refactoring if statements right now won't make huge improvements for the code.

[elfring]

:bulb: I would generally prefer to reduce code duplication.