Round duration when saving animated WebP images

[radarhere]

Helps #7015

Alternative to #6977. This uses round() instead of int() to better approximate the data, and has a test using one of our existing images.

Animated PNGs can have float durations

Pillow/src/PIL/PngImagePlugin.py

Line 677 in cdf5fd4

self.im_info["duration"] = float(delay_num) / float(delay_den) * 1000

but when WebP goes to save the duration, it expects an integer.

Pillow/src/PIL/WebPImagePlugin.py

Line 308 in cdf5fd4

enc.add(None, timestamp, 0, 0, "", lossless, quality, 0)

Pillow/src/_webp.c

Lines 206 to 209 in cdf5fd4

	"z#iiisifi",
	(char **)&rgb,
	&size,
	&timestamp,

The WebP API itself also expects the duration to be an integer. See https://developers.google.com/speed/webp/docs/container-api#webpanimencoderadd

int WebPAnimEncoderAdd(
WebPAnimEncoder* enc, struct WebPPicture* frame, int timestamp_ms,
const struct WebPConfig* config);

So this PR rounds the duration.