[feature] Enable basic video support (mp4 only) (#1274)

* [feature] basic video support

* fix missing semicolon

* replace text shadow with stacked icons

Co-authored-by: f0x <f0x@cthu.lu>

vendor/github.com/abema/go-mp4/write.go

@@ -0,0 +1,68 @@
+package mp4
+
+import (
+	"errors"
+	"io"
+)
+
+type Writer struct {
+	writer  io.WriteSeeker
+	biStack []*BoxInfo
+}
+
+func NewWriter(w io.WriteSeeker) *Writer {
+	return &Writer{
+		writer: w,
+	}
+}
+
+func (w *Writer) Write(p []byte) (int, error) {
+	return w.writer.Write(p)
+}
+
+func (w *Writer) Seek(offset int64, whence int) (int64, error) {
+	return w.writer.Seek(offset, whence)
+}
+
+func (w *Writer) StartBox(bi *BoxInfo) (*BoxInfo, error) {
+	bi, err := WriteBoxInfo(w.writer, bi)
+	if err != nil {
+		return nil, err
+	}
+	w.biStack = append(w.biStack, bi)
+	return bi, nil
+}
+
+func (w *Writer) EndBox() (*BoxInfo, error) {
+	bi := w.biStack[len(w.biStack)-1]
+	w.biStack = w.biStack[:len(w.biStack)-1]
+	end, err := w.writer.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return nil, err
+	}
+	bi.Size = uint64(end) - bi.Offset
+	if _, err = bi.SeekToStart(w.writer); err != nil {
+		return nil, err
+	}
+	if bi2, err := WriteBoxInfo(w.writer, bi); err != nil {
+		return nil, err
+	} else if bi.HeaderSize != bi2.HeaderSize {
+		return nil, errors.New("header size changed")
+	}
+	if _, err := w.writer.Seek(end, io.SeekStart); err != nil {
+		return nil, err
+	}
+	return bi, nil
+}
+
+func (w *Writer) CopyBox(r io.ReadSeeker, bi *BoxInfo) error {
+	if _, err := bi.SeekToStart(r); err != nil {
+		return err
+	}
+	if n, err := io.CopyN(w, r, int64(bi.Size)); err != nil {
+		return err
+	} else if n != int64(bi.Size) {
+		return errors.New("failed to copy box")
+	}
+	return nil
+}