NV12 allow NULL for Y

docs/formats.md

 # Introduction
 
 Formats (FOURCC) supported by libyuv are detailed here.
 
 # Core Formats
 
 There are 2 core formats supported by libyuv - I420 and ARGB.  All YUV formats can be converted to/from I420.  All RGB formats can be converted to/from ARGB.
 
 Filtering functions such as scaling and planar functions work on I420 and/or ARGB.
 
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       FOURCC_BGR3 = FOURCC('B', 'G', 'R', '3'),  // Alias for 24BG.
       FOURCC_CM32 = FOURCC(0, 0, 0, 32),  // Alias for BGRA kCMPixelFormat_32ARGB
       FOURCC_CM24 = FOURCC(0, 0, 0, 24),  // Alias for RAW kCMPixelFormat_24RGB
       FOURCC_L555 = FOURCC('L', '5', '5', '5'),  // Alias for RGBO.
       FOURCC_L565 = FOURCC('L', '5', '6', '5'),  // Alias for RGBP.
       FOURCC_5551 = FOURCC('5', '5', '5', '1'),  // Alias for RGBO.
 
       // 1 Auxiliary compressed YUV format set aside for capturer.
       FOURCC_H264 = FOURCC('H', '2', '6', '4'),
 
+# Planar YUV
+      The following formats contains a full size Y plane followed by 1 or 2
+        planes for UV: I420, I422, I444, I411, I400, NV21, NV12, I400
+      The size (subsampling) of the UV varies.
+        I420, NV12 and NV21 are half width, half height
+        I422, NV16 and NV61 are half width, full height
+        I444, NV24 and NV42 are full width, full height
+        I400 and J400 have no chroma channel.
+
 # The ARGB FOURCC
 
 There are 4 ARGB layouts - ARGB, BGRA, ABGR and RGBA.  ARGB is most common by far, used for screen formats, and windows webcam drivers.
 
 The fourcc describes the order of channels in a ***register***.
 
 A fourcc provided by capturer, can be thought of string, e.g. "ARGB".
 
 On little endian machines, as an int, this would have 'A' in the lowest byte.  The FOURCC macro reverses the order:
 
     #define FOURCC(a, b, c, d) (((uint32)(a)) | ((uint32)(b) << 8) | ((uint32)(c) << 16) | ((uint32)(d) << 24))
 
 So the "ARGB" string, read as an uint32, is
 
     FOURCC_ARGB = FOURCC('A', 'R', 'G', 'B')
 
 If you were to read ARGB pixels as uint32's, the alpha would be in the high byte, and the blue in the lowest byte.  In memory, these are stored little endian, so 'B' is first, then 'G', 'R' and 'A' last.
 
 When calling conversion functions, the names match the FOURCC, so in this case it would be I420ToARGB().
 
 All formats can be converted to/from ARGB.
 
 Most 'planar_functions' work on ARGB (e.g. ARGBBlend).
 
 Some are channel order agnostic (e.g. ARGBScale).
 
 Some functions are symmetric (e.g. ARGBToBGRA is the same as BGRAToARGB, so its a macro).
 
 ARGBBlend expects preattenuated ARGB. The R,G,B are premultiplied by alpha.  Other functions don't care.