Multiple-resolution encoder

The example encoder down-samples the input video frames a number of
times with a down-sampling factor, and then encodes and outputs
bitstreams with different resolutions.

Support arbitrary down-sampling factor, and down-sampling factor
can be different for each encoding level.

For example, the encoder can be tested as follows.
1. Configure with multi-resolution encoding enabled:
../libvpx/configure --target=x86-linux-gcc --disable-codecs
--enable-vp8 --enable-runtime_cpu_detect --enable-debug
--disable-install-docs --enable-error-concealment
--enable-multi-res-encoding
2. Run make
3. Encode:
If input video is 1280x720, run:
./vp8_multi_resolution_encoder 1280 720 input.yuv 1.ivf 2.ivf 3.ivf 1
(output: 1.ivf(1280x720); 2.ivf(640x360); 3.ivf(320x180).
The last parameter is set to 1/0 to show/not show PSNR.)
4. Decode:
./simple_decoder 1.ivf 1.yuv
./simple_decoder 2.ivf 2.yuv
./simple_decoder 3.ivf 3.yuv
5. View video:
mplayer 1.yuv -demuxer rawvideo -rawvideo w=1280:h=720 -loop 0 -fps 30
mplayer 2.yuv -demuxer rawvideo -rawvideo w=640:h=360 -loop 0 -fps 30
mplayer 3.yuv -demuxer rawvideo -rawvideo w=320:h=180 -loop 0 -fps 30

The encoding parameters can be modified in vp8_multi_resolution_encoder.c,
for example, target bitrate, frame rate...

Modified API. John helped a lot with that. Thanks!

Change-Id: I03be9a51167eddf94399f92d269599fb3f3d54f5

configure

@@ -35,7 +35,7 @@ Advanced options:
   ${toggle_internal_stats}        output of encoder internal stats for debug, if supported (encoders)
   ${toggle_mem_tracker}           track memory usage
   ${toggle_postproc}              postprocessing
-  ${toggle_multithread}           multithreaded encoding and decoding.
+  ${toggle_multithread}           multithreaded encoding and decoding
   ${toggle_spatial_resampling}    spatial sampling (scaling) support
   ${toggle_realtime_only}         enable this option while building for real-time encoding
   ${toggle_error_concealment}     enable this option to get a decoder which is able to conceal losses
@@ -44,6 +44,7 @@ Advanced options:
   ${toggle_static}                static library support
   ${toggle_small}                 favor smaller size over speed
   ${toggle_postproc_visualizer}   macro block / block level visualizers
+  ${toggle_multi_res_encoding}    enable multiple-resolution encoding
 
 Codecs:
   Codecs can be selectively enabled or disabled individually, or by family:
@@ -262,6 +263,7 @@ CONFIG_LIST="
     postproc_visualizer
     os_support
     unit_tests
+    multi_res_encoding
 "
 CMDLINE_SELECT="
     extra_warnings
@@ -304,6 +306,7 @@ CMDLINE_SELECT="
     small
     postproc_visualizer
     unit_tests
+    multi_res_encoding
 "
 
 process_cmdline() {