mpegvideo: Move QUANT_BIAS_SHIFT define to the only place it is used
fft-test: Make global exptab variable static
mpegenccontext: Remove unused dct_unquantize_h261 function pointers

Signed-off-by: Diego Biurrun <diego@biurrun.de>
fft-test: Reduce variable scope where sensibly possible

Also merge variable declaration and initialization.
fft-test: Drop unnecessary pointer indirection for context structs