Reduce allocations of row buffers to 1 alloc/free.
Do 2 rows at a time to avoid converting U and V planes twice.

Bug: libyuv:715
Test: LibYUVConvertTest.H010ToAR30_Opt
Change-Id: I2f3a03b4875df5e3b969112a78a1a0b28399fa2f
Reviewed-on: https://chromium-review.googlesource.com/816021Reviewed-by: Cheng Wang <wangcheng@google.com>