Update source to SDL2 2.0.12

Edward Rudd

2020-05-02 03f8528315fa46c95991a34f3325d7b33ae5538c

 source/src/libm/s_atan.c

@@ -60,6 +60,10 @@
  1.62858201153657823623e-02, /* 0x3F90AD3A, 0xE322DA11 */
};

#ifdef __WATCOMC__ /* Watcom defines huge=__huge */
#undef huge
#endif

static const double
one   = 1.0,
huge   = 1.0e300;