vpxenc: Add support for pixel aspect ratio.

WebM files will adjust the display width and height according to the
input pixel aspect ratio. The default pixel aspect ratio is 1:1.

BUG=https://code.google.com/p/webm/issues/detail?id=1005

Change-Id: I23e0a601b7259fa9513cb86110c41b8437769808

tools_common.h

@@ -89,6 +89,7 @@ struct VpxInputContext {
   enum VideoFileType file_type;
   uint32_t width;
   uint32_t height;
+  struct VpxRational pixel_aspect_ratio;
   vpx_img_fmt_t fmt;
   vpx_bit_depth_t bit_depth;
   int only_i420;