Fix the last std::move-error when building with clang.
Fix the last std::move-error when building with clang.

Clang reports the following prior to applying this patch:
xyuv/xyuv/src/external/libpng_wrapper.cpp:295:21: error: moving a temporary object prevents copy elision [-Werror,-Wpessimizing-move]

stian-svedenborg/xyuv,stian-svedenborg/xyuv,stian-svedenborg/xyuv