optimize text rendering

benchmark                  old ns/op     new ns/op     delta
BenchmarkRender480p        10748199      8731322       -18.76%
BenchmarkRender480pSS      16353700      12750238      -22.03%
BenchmarkRender720p        19845388      16929130      -14.69%
BenchmarkRender720pSS      32514554      27781420      -14.56%
BenchmarkRender1080p       35496819      30735670      -13.41%
BenchmarkRender1080pSS     53060910      46518445      -12.33%
BenchmarkRender4K          115428564     103907287     -9.98%
BenchmarkRender4KSS        148652603     129781380     -12.69%

benchmark                  old allocs     new allocs     delta
BenchmarkRender480p        223            44             -80.27%
BenchmarkRender480pSS      580            150            -74.14%
BenchmarkRender720p        223            44             -80.27%
BenchmarkRender720pSS      584            150            -74.32%
BenchmarkRender1080p       223            44             -80.27%
BenchmarkRender1080pSS     584            150            -74.32%
BenchmarkRender4K          223            44             -80.27%
BenchmarkRender4KSS        586            150            -74.40%

benchmark                  old bytes     new bytes     delta
BenchmarkRender480p        257978        46167         -82.10%
BenchmarkRender480pSS      467718        168033        -64.07%
BenchmarkRender720p        266169        50263         -81.12%
BenchmarkRender720pSS      709359        176225        -75.16%
BenchmarkRender1080p       276414        55383         -79.96%
BenchmarkRender1080pSS     726771        188521        -74.06%
BenchmarkRender4K          312305        73309         -76.53%
BenchmarkRender4KSS        1301899       221307        -83.00%