texturator: Fix float to bool conversion warning

The following build warning is seen:

../texturator.c:603:45: warning: '*' in boolean context, suggest '&&' instead [-Wint-in-bool-context]
  *complemented = (((float)rgba[2]) / 255.0) / 0.25;

Fix it by using the != 0.0f construction to properly convert from float
to bool.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
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