imgutils: add function to clear an image to black
Black isn't always just memset(ptr, 0, size). Limited YUV in particular requires relatively non-obvious values, and filling a frame with repeating 0 bytes is disallowed in some contexts. With component sizes larger than 8 or packed YUV, this can become relatively complicated. So having a generic function for this seems helpful. In order to handle the complex cases in a generic way without destroying performance, this code attempts to compute a black pixel, and then uses that value to clear the image data quickly by using a function like memset. Common cases like yuv410p10 or rgba can't be handled with a simple memset, so there is some code to fill memory with 2/4/8 byte patterns. For the remaining cases, a generic slow fallback is used. Signed-off-by:Anton Khirnov <anton@khirnov.net> Merged from Libav commit 45df7adc.
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