video: silence warnings for missing libavutils pixel formats

Using vf_screenshot on Libav printed useless/misleading error messages
when playing 10 bit h264 with a VO that supports 8 bit yuv420p only:

    Unsupported format 444p14le
    Unsupported format 444p14be
    ...

The cause of this is that vf_scale is inserted to handle the format
conversion, and tries to find a pixel format with best quality. This
includes the 14 bit and 12 bit formats, which don't exist on Libav.
vf_screenshot tries to query whether Libav's libswscale supports it,
resulting in these error messages. (In theory, vf_scale is missing this
check, but it doesn't matter in practice.)

Since this warning is rather useless anyway, because all input video
comes from libavcodec, and only the conversion into the other could
possibly fail. Silence the warning by raising it to verbose message
level.

Closes #7.

video/fmt-conversion.c

@@ -139,7 +139,7 @@ enum PixelFormat imgfmt2pixfmt(int fmt)
             break;
     pix_fmt = conversion_map[i].pix_fmt;
     if (pix_fmt == PIX_FMT_NONE)
-        mp_msg(MSGT_GLOBAL, MSGL_ERR, "Unsupported format %s\n", vo_format_name(fmt));
+        mp_msg(MSGT_GLOBAL, MSGL_V, "Unsupported format %s\n", vo_format_name(fmt));
     return pix_fmt;
 }