Add support in VideoFrame ndarray for gbrpf32 format

Created by: corco

Adds support for floating point RGB format.

Can be tested with this script:

import numpy as np
import av


WIDTH = 1920
HEIGHT = 1080
BIT_DEPTH = 12

if BIT_DEPTH == 8:
    pix_fmt = "gbrp"
    dtype = np.uint8
else:
    pix_fmt = f"gbrp{BIT_DEPTH}le"
    dtype = np.uint16

ocntnr = av.open("test.mp4", "w")
ovstrm = ocntnr.add_stream("av1")
ovstrm.width = WIDTH
ovstrm.height = HEIGHT
ovstrm.pix_fmt = pix_fmt

max = 2**BIT_DEPTH - 1
gradient = np.repeat(np.arange(WIDTH) / WIDTH, 3)
h = np.round(np.linspace(0, HEIGHT, 9, endpoint=True)).astype(np.uint32)

colours = [
    [max, max, max],  # White
    [max, 0, 0],  # Red
    [0, max, 0],  # Green
    [0, 0, max],  # Blue
    [max, max, 0],  # Yellow
    [max, 0, max],  # Magenta
    [0, max, max],  # Cyan
    [0, 0, 0],  # Black
]

array = np.empty((HEIGHT, WIDTH, 3), dtype=dtype)
for idx, colour in enumerate(colours):
    array[h[idx] : h[idx + 1], :, :] = (gradient * np.tile(np.array(colour, dtype=dtype), WIDTH)).reshape(WIDTH, -1)

frame = av.VideoFrame.from_ndarray(array, pix_fmt)
for p in ovstrm.encode(frame):
    ocntnr.mux(p)

for p in ovstrm.encode():
    ocntnr.mux(p)

ocntnr.close()