Tests/LibVideo: Test to ensure that VP9 reference vector clamping works

The test file is a short clip from Big Buck Bunny, so a license file
was added to the directory to attribute it.

Tests/LibVideo/CMakeLists.txt

@@ -8,3 +8,4 @@ endforeach()
 
 install(FILES vp9_in_webm.webm DESTINATION usr/Tests/LibVideo)
 install(FILES vp9_4k.webm DESTINATION usr/Tests/LibVideo)
+install(FILES vp9_clamp_reference_mvs.webm DESTINATION usr/Tests/LibVideo)

Tests/LibVideo/FILE_LICENSES.md

@@ -0,0 +1,4 @@
+## vp9_clamp_reference_mvs.webm
+
+Licensed under [Creative Commons Attribution 3.0](http://creativecommons.org/licenses/by/3.0/)\
+(c) copyright 2008, Blender Foundation / [www.bigbuckbunny.org](https://www.bigbuckbunny.org)

Tests/LibVideo/TestVP9Decode.cpp

@@ -49,3 +49,8 @@ BENCHMARK_CASE(vp9_4k)
 {
     decode_video("./vp9_4k.webm"sv, 2);
 }
+
+BENCHMARK_CASE(vp9_clamp_reference_mvs)
+{
+    decode_video("./vp9_clamp_reference_mvs.webm"sv, 92);
+}