vo_corevideo: restructure this video output

vo_corevideo: restructure this video output

Restructure this video output to be similar to vo_gl, even if simpler
and less feature complete (for example it's still missing EOSD
support). Ideally, it should act as a decent fallback in the case
where something breaks in the OSX support of vo_gl.

Here's a summary of what changed:

 * Remove the shared buffer code since it wasn't using any function
   from the CoreVideo API. Moreover, its presence in vo_corevideo was
   forcing the non-GUI related code to perform more image copies than
   necessary. Equivalent shared-buffer functionality will be added in
   a separate new VO in the next commit (this means OSX GUIs will need
   to specify a different VO).

 * Clean up the code to conform a bit more to the mplayer2
   conventions. Enforce 80 column wrapping, use a private struct for
   file variables, use the new libvo api.

 * Add OSD rendering using OpenGL instead of writing directly on the
   video image data.

 * Simplify the logic for the rendering function when dealing with
   panscan.

 * Add VOCTRL_REDRAW_FRAME support.

 * Add colormatrix support by using the built-in API provided by
   CoreVideo.