avfilter/vf_fspp: use M_SQRT1_2, M_SQRT2

This uses M_SQRT1_2, M_SQRT2 instead of the actual literals. Fixed point
values remain unchanged.
Patch tested with FATE on x86.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavfilter/vf_fspp.h

@@ -44,12 +44,12 @@
 typedef int32_t int_simd16_t;
 static const int16_t FIX_0_382683433   = FIX(0.382683433, 14);
 static const int16_t FIX_0_541196100   = FIX(0.541196100, 14);
-static const int16_t FIX_0_707106781   = FIX(0.707106781, 14);
+static const int16_t FIX_0_707106781   = FIX(M_SQRT1_2  , 14);
 static const int16_t FIX_1_306562965   = FIX(1.306562965, 14);
-static const int16_t FIX_1_414213562_A = FIX(1.414213562, 14);
+static const int16_t FIX_1_414213562_A = FIX(M_SQRT2    , 14);
 static const int16_t FIX_1_847759065   = FIX(1.847759065, 13);
 static const int16_t FIX_2_613125930   = FIX(-2.613125930, 13);
-static const int16_t FIX_1_414213562   = FIX(1.414213562, 13);
+static const int16_t FIX_1_414213562   = FIX(M_SQRT2    , 13);
 static const int16_t FIX_1_082392200   = FIX(1.082392200, 13);
 
 typedef struct FSPPContext {