先Y后U后V，无冗余信息
A video sequence stored in the YUV format is stored by concatenating
consecutive frames into a single big file. Each frame contains the Y pixels
for that frame first, followed by the U pixels (actually Cr pixel), followed
by V pixels (actually Cb pixels). Each component is raster-scanned from
the top left corner down to the bottom right corner.
Each pixel value is stored as a 8 bit unsigned character.
For example, a frame in a QCIF sequence with 176x144 Y pixels and
88x72 U and V pixels is stored in the following order:
176
+-------------+
| |
| Y | 144
| |
| |
+-------------+
88
+------+
| U |
| |72
+------+
| V |
| |72
+------+
