perf(Mp4Generator): assemble segment data in a single allocation

[vanyaxk]

This PR reduces allocation pressure in MP4 segment generation by writing mdat data directly into the final segment buffer - this avoids one full media-payload copy per segment

[shaka-bot]

Bundle Size Report for PR #10101

File	HEAD	Base	Diff
controls.css	24.0 KiB (5.0 KiB)	24.0 KiB (5.0 KiB)	+0.0 KiB
demo.compiled.debug.js	258.3 KiB (51.8 KiB)	258.3 KiB (51.8 KiB)	0.0 KiB
demo.compiled.js	258.3 KiB (51.8 KiB)	258.3 KiB (51.8 KiB)	0.0 KiB
demo.css	170.4 KiB (27.8 KiB)	170.2 KiB (27.8 KiB)	+0.1 KiB
locales.js	43.6 KiB (10.8 KiB)	43.6 KiB (10.8 KiB)	0.0 KiB
receiver.compiled.debug.js	148.8 KiB (24.1 KiB)	148.8 KiB (24.1 KiB)	0.0 KiB
receiver.compiled.js	148.8 KiB (24.1 KiB)	148.8 KiB (24.1 KiB)	0.0 KiB
shaka-player.compiled-es2021.debug.js	1258.7 KiB (309.6 KiB)	1258.4 KiB (309.5 KiB)	+0.3 KiB
shaka-player.compiled-es2021.js	666.6 KiB (222.4 KiB)	666.4 KiB (222.3 KiB)	+0.2 KiB
shaka-player.compiled.debug.js	1530.8 KiB (350.8 KiB)	1530.5 KiB (350.7 KiB)	+0.3 KiB
shaka-player.compiled.js	770.7 KiB (250.1 KiB)	770.4 KiB (250.0 KiB)	+0.3 KiB
shaka-player.dash-es2021.debug.js	926.1 KiB (232.8 KiB)	925.7 KiB (232.6 KiB)	+0.4 KiB
shaka-player.dash-es2021.js	460.0 KiB (157.0 KiB)	459.9 KiB (157.0 KiB)	+0.1 KiB
shaka-player.dash.debug.js	1124.1 KiB (263.0 KiB)	1123.6 KiB (262.7 KiB)	+0.5 KiB
shaka-player.dash.js	534.2 KiB (177.0 KiB)	534.1 KiB (177.0 KiB)	+0.1 KiB
shaka-player.experimental-es2021.debug.js	1649.8 KiB (394.2 KiB)	1649.5 KiB (394.1 KiB)	+0.3 KiB
shaka-player.experimental-es2021.js	919.6 KiB (291.5 KiB)	919.4 KiB (291.4 KiB)	+0.2 KiB
shaka-player.experimental.debug.js	1986.2 KiB (444.4 KiB)	1985.9 KiB (444.3 KiB)	+0.3 KiB
shaka-player.experimental.js	1045.7 KiB (325.0 KiB)	1045.4 KiB (324.9 KiB)	+0.3 KiB
shaka-player.hls-es2021.debug.js	956.3 KiB (238.2 KiB)	956.1 KiB (238.1 KiB)	+0.3 KiB
shaka-player.hls-es2021.js	493.1 KiB (167.1 KiB)	492.9 KiB (167.1 KiB)	+0.2 KiB
shaka-player.hls.debug.js	1159.5 KiB (269.1 KiB)	1159.2 KiB (268.9 KiB)	+0.3 KiB
shaka-player.hls.js	570.2 KiB (188.1 KiB)	569.9 KiB (188.0 KiB)	+0.3 KiB
shaka-player.ui-es2021.debug.js	1567.3 KiB (376.8 KiB)	1567.0 KiB (376.7 KiB)	+0.3 KiB
shaka-player.ui-es2021.js	879.1 KiB (280.0 KiB)	878.9 KiB (279.9 KiB)	+0.2 KiB
shaka-player.ui.debug.js	1873.7 KiB (423.1 KiB)	1873.4 KiB (423.0 KiB)	+0.3 KiB
shaka-player.ui.js	993.7 KiB (310.7 KiB)	993.4 KiB (310.7 KiB)	+0.3 KiB

[shaka-bot]

Incremental code coverage: 100.00%