io,EmitRescaledAlphaYUV: factor out a common expr
io,EmitRescaledAlphaYUV: factor out a common expr

+ s/src_a/dst_a/
+ remove unnecessary (void) as expected_num_lines_out is used within the
  function

Change-Id: Ic45f798ef22bd19eaabf1a0512d1cf8a201bb4b5

PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp