Aligned the image buffer and stride to 32 bytes. This enables
calling of optimized scaler function in libyuv, and improves
the performance.

Tested libyuv scaler(x86 optimization) on Linux and Windows,
including: Linux 32/64bit, visual studio 32/64bit, Cygwin, and
MinGW32.

Also, fixed a wrong pointer in vpx_codec_encode().

Change-Id: Ibe97d7a0a745f82c43852fa4ed719be5a4db6abc