Reimplemented the blend filter in C++ and completed the colorspaces supported.

It now supports all combinaisons of
Destinations:
 - YV9, YV12, I410, I411 (8 bits)
 - I420, I422, I444 (8, 9 and 10 bits, native endianness only)
 - YUYV and the various variants
 - NV12/NV21
 - RV15, RV16, RV24, RV32
Sources
 - YUVA
 - RGBA
 - YUVP

The quality of the blending is the same, and the speeds for I420 is the same
(not tested others combinaisons).

Support for blending I420 over another picture has been removed (I don't
see the use case and it simplify the code).

It closes ##5477.

modules/video_filter/Modules.am

@@ -21,7 +21,7 @@ SOURCES_deinterlace = deinterlace/deinterlace.c deinterlace/deinterlace.h \
 	deinterlace/yadif.h deinterlace/yadif_template.h \
 	deinterlace/algo_phosphor.c deinterlace/algo_phosphor.h \
 	deinterlace/algo_ivtc.c deinterlace/algo_ivtc.h
-SOURCES_blend = blend.c
+SOURCES_blend = blend.cpp
 SOURCES_scale = scale.c
 SOURCES_marq = marq.c
 SOURCES_rss = rss.c