fix(darwin): fix mpv_render_context_free call #156

Job	Run time
package:media_kit tests (Linux)	36m 20s
Android	6m 2s
iOS	2m 43s
macOS	7m 8s
package:media_kit tests (Web)	30m 46s
Windows (optional libs)	4m 53s
Windows	7m 23s
Linux (audio libs)	2m 34s
macOS (audio libs)	4m 19s
Windows (audio libs)	4m 51s
Android (optional libs)	6m 16s
iOS (optional libs)	7m 12s
Linux (optional libs)	3m 23s
macOS (optional libs)	4m 7s
Android (audio libs)	6m 12s
Linux	2m 36s
package:media_kit tests (Windows)	37m 9s
iOS (audio libs)	2m 44s
	2h 56m 38s

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