[Impeller] write glyphs to malloc buffer.

[jonahwilliams]

Writing to a malloc buffer and then copying is faster on a variety of Pixel devices. Since this change was supposed to be a performance optimization, just back it out.

[flutter-dashboard]

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[jonahwilliams]

Goal is to fix this regression: https://flutter-flutter-perf.skia.org/e/?queries=device_type%3DPixel_7_Pro%26sub_result%3D90th_percentile_frame_rasterizer_time_millis%26sub_result%3D99th_percentile_frame_rasterizer_time_millis%26sub_result%3Daverage_frame_rasterizer_time_millis%26sub_result%3Dworst_frame_rasterizer_time_millis%26test%3Dcomplex_layout_scroll_perf_impeller__timeline_summary&selected=commit%3D40761%26name%3D%252Carch%253Dintel%252Cbranch%253Dmaster%252Cconfig%253Ddefault%252Cdevice_type%253DPixel_7_Pro%252Cdevice_version%253Dnone%252Chost_type%253Dlinux%252Csub_result%253Dworst_frame_rasterizer_time_millis%252Ctest%253Dcomplex_layout_scroll_perf_impeller__timeline_summary%252C

[jonahwilliams]

Fixes flutter/flutter#147996

(Not because we did it, because we don't need it)