avfilter/vsrc_mandelbrot: change sin to sinf for color computation

lrintf is anyway used, suggesting we only care up to floating precision.
Rurthermore, there is a compat hack in avutil/libm for this function,
and it is used in avcodec/aacps_tablegen.h.

This yields a non-negligible speedup. Sample benchmark:
x86-64, Haswell, GNU/Linux:

old (draw_mandelbrot):
274635709 decicycles in draw_mandelbrot,     256 runs,      0 skips
300287046 decicycles in draw_mandelbrot,     512 runs,      0 skips
371819935 decicycles in draw_mandelbrot,    1024 runs,      0 skips
336663765 decicycles in draw_mandelbrot,    2048 runs,      0 skips
581851016 decicycles in draw_mandelbrot,    4096 runs,      0 skips

new (draw_mandelbrot):
269882717 decicycles in draw_mandelbrot,     256 runs,      0 skips
296359285 decicycles in draw_mandelbrot,     512 runs,      0 skips
370076599 decicycles in draw_mandelbrot,    1024 runs,      0 skips
331478354 decicycles in draw_mandelbrot,    2048 runs,      0 skips
571904318 decicycles in draw_mandelbrot,    4096 runs,      0 skips

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
