get rid of unused entries in the middle of xvmc_render struct,

extensions are added to the end of the struct (avoids build issues on 64 bits platform)
add osd extension to be used by mplayer (required by the resent changes in the build system)
breaking the compability, thus bumping minor lavc version number

Originally committed as revision 8527 to svn://svn.ffmpeg.org/ffmpeg/trunk