memory: Add 0 padding
Change gst_allocator_alloc() so that we can also spicify flags and padding. Add 2 new flags to mark the memory 0 prefixed/padded. This allows us to remove some resizes in the base classes. When allocating memory, memset prefix and padding with 0 when the flags tell us to. On resize, clear the zero padding flags if we can't guarantee the memory is still 0 filled. Update tests.
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