Add the initial libyuv support

The main benefit of libyuv, apart from shortening the video pipeline, is quite noticeable latency and CPU usage decrease due to various assembler/SIMD optimizations of the library. However, there is a drawback for macOS systems: libyuv cannot be downloaded as a compiled library and can only be built from the source, which means we should include a cropped source code of the library (~10K LoC) into the app or rise the complexity of macOS dev and run toolchains. The main target system -- Linux, and Windows will use compiled lib from the package managers and macOS will use the lib included as a shortened source-code.

Building the app with the no_libyuv tag will force it to use libyuv from the provided source files.

test/test.go

@@ -0,0 +1,17 @@
+package test
+
+import (
+	"os"
+	"path"
+	"runtime"
+)
+
+// runs tests from the root dir when imported
+
+func init() {
+	_, filename, _, _ := runtime.Caller(0)
+	dir := path.Join(path.Dir(filename), "..")
+	if err := os.Chdir(dir); err != nil {
+		panic(err)
+	}
+}