nuv: Reuse the DSPContext from RTJpegContext

There is no point in populating NuvContext with another DSPContext.

Also split static and dynamic initialization bits to avoid running the
static initialization parts over and over.
x86: dsputil: Move inline assembly macros to a separate header
imgconvert: Move ff_deinterlace_line_*_mmx declarations out of dsputil
dsputil: Move ff_zigzag_direct and ff_crop_tab declarations to mathops.h
dsputil: Refactor duplicated CALL_2X_PIXELS / PIXELS16 macros
x86: dsputil: Move fpel declarations to a separate header
ppc: dsputil: Move hpeldsp-related declarations to a separate header
x86: dsputil: Move hpeldsp-related declarations to a separate header
dsputil: Move thirdpel-related bits into their own context