mm-video: venc: use source stride for color-converting RGB input

 -RGB input may be composed by GPU with an aligned stride
  different from the frame-width.
 -CTS test cases testEncodeDecodeVideoFromSurfaceToSurface720p
  and testVideoEdit720p fails because gralloc-handle
  stride value is not used
 -Extract this stride from gralloc-handle to interpret the buffer

Change-Id: I63fd224e933424e1e8c49a19270545f000a2ebd5