Merge changes I1425f12d,I1e9e9ffa,I6d8f676b,I92013086 into main

* changes:
  mem_sse2.h: loadu_uint32 -> loadu_int32
  mem_sse2.h: storeu_uint32 -> storeu_int32
  vp8: fix some implicit signed -> unsigned conv warnings
  video_source.h,ReallocImage: quiet implicit conv warning