Add support for VideoEncParams and VideoBlockParams

[benedikt-grl]

FFmpeg allows accessing the quantization parameters for a few video codecs via side data. This PR adds support for accessing the VideoEncParams and VideoBlockParams.
Something similar was proposed in #779, but has not been implemented yet.

After setting stream.codec_context.options = {"export_side_data": "venc_params"}, the user can do something like this:

if sd.type == av.sidedata.sidedata.Type.VIDEO_ENC_PARAMS:
    for block_idx in range(sd.nb_blocks):
        b = sd.block_params(block_idx)
        # b has properties src_x, src_y, w, h, delta_qp

I also added a convenience method sd.qp_map() that arranges the quantization parameters in a 2-D grid. The quantization parameters are calculated as sd.qp + b.delta_qp, analogously to ff_qp_table_extract.

Full example:

import av
import matplotlib.pyplot as plt
import numpy as np


def draw_qp_map(qp_map):
    # Determine figure size
    map_height, map_width = qp_map.shape
    width_px = map_width * 16
    height_px = map_height * 16
    font_size = 7

    # Create figure
    fig, ax = plt.subplots(1, 1, figsize=(width_px / 50, height_px / 50))

    # Display QP map
    ax.imshow(qp_map, vmin=0, vmax=51)

    # Annotate each macroblock with its quantization parameter
    for y in range(map_height):
        for x in range(map_width):
            ax.text(x, y, qp_map[y, x], horizontalalignment="center", verticalalignment="center", color="black", fontsize=font_size)

    # Show macroblock coordinates
    ax.set_xticks(np.arange(map_width))
    ax.set_yticks(np.arange(map_height))

    # Show macroblock indices at all sides
    ax.tick_params(
        top=True,
        bottom=True,
        left=True,
        right=True,
        labeltop=True,
        labelbottom=True,
        labelright=True,
    )

    return fig, ax


video_filepath = "video.mp4"
container = av.open(video_filepath)

stream = container.streams.video[0]
stream.codec_context.options = {"export_side_data": "venc_params"}

frame_iter = container.decode(video=0)

# Select frame by index
frame_idx = 0

# Skip ahead to the frame of interest
for i in range(frame_idx + 1):
    frame = next(frame_iter)

qp_map = None
for sd in frame.side_data:
    if sd.type == av.sidedata.sidedata.Type.VIDEO_ENC_PARAMS:
        qp_map = sd.qp_map()

fig, ax = draw_qp_map(qp_map)
plt.show()