FlowFPS: Request additional frames only for mask=2
PlaneOfBlocks: Make the function selection nicer

Also makes it possible to support more subsamplings with few changes to
the function selection.
Bring back the SATD functions

In preparation for the return of the "dct" parameter.
Fix for DCTFFTW. Return parameter "dct".
Sanity checks for dct parameter
Update the build system

FFTW is required once again.