sws_getContext,sws_scale

  最大支持的圖像大小，sws_getContext 源碼中：

 00839     if(srcW > VOFW || dstW > VOFW) {
00840         av_log(NULL, AV_LOG_ERROR, "swScaler: Compile-time maximum width is "AV_STRINGIFY(VOFW)" change VOF/VOFW and recompile\n");
00841         return NULL;
00842     }

#define VOFW   2048

http://ffmpeg.org/doxygen/0.6/libswscale_2utils_8c-source.html#l00782