Move platform-specific code under build tags; fix corresponding tests

ffmpeg_windows_test.go

@@ -0,0 +1,20 @@
+package ffmpeg_go
+
+import (
+	"os/exec"
+	"syscall"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCompileWithOptions(t *testing.T) {
+	out := Input("dummy.mp4").Output("dummy2.mp4")
+	cmd := out.Compile(func(s *Stream, cmd *exec.Cmd) {
+		if cmd.SysProcAttr == nil {
+			cmd.SysProcAttr = &syscall.SysProcAttr{}
+		}
+		cmd.SysProcAttr.HideWindow = true
+	})
+	assert.Equal(t, true, cmd.SysProcAttr.HideWindow)
+}