lavfi: rename decimate to mpdecimate.

The next commit will introduce a proper decimation filter to be used
along with the field matching filter. To avoid confusion with this
filter which has currently a very limited usage (and will not work
properly with the fieldmatching filter), the new decimation filter will
take the decimate name, and this filter is renamed to mpdecimate.
lavfi: port IVTC filters from vapoursynth.
doc/filters: move mpdecimate doc to a more appropriate position.
sws/output: add yuv2rgb_full_1_c_template()

7500->4500 cycles on sandybridge

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/output: fix alpha scale in the new functions

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>