Speed up some conversions

[akx]

Local results for the touched conversions:

------------------------- benchmark 'convert': 28 tests, 2 sources ------------------------
Name (time in us)                              0001_7d5d2c3 Min  0002_643f45b Min      ΔMin
-------------------------------------------------------------------------------------------
test_convert[1024x1024-I-L]                             96.9160           55.9581    -42.3%
test_convert[1024x1024-P-RGB]                          846.0840          478.4579    -43.5%
test_convert[1024x1024-PA-RGB]                         854.7499          497.6669    -41.8%
test_convert[1024x1024-PA-RGBA]                        994.4162          520.9171    -47.6%
test_convert[1024x1024-P-RGBA]                       1,064.0421          407.7090    -61.7%
-------------------------------------------------------------------------------------------

Experiments to speed up the other conversions didn't really yield fruit.