Add -ftrapping-math to default cflags

This should resolve pixman/pixman#22
and make the tests pass with clang.

-ftrapping-math is already the default[1] for gcc, so this should not change
behavior when compiling with gcc.  However, clang defaults[2] to -fno-trapping-math,
so -ftrapping-math is needed to avoid floating-point expceptions when running the
combiner and stress tests.

The root causes of this issue is that that pixman-combine-float.c guards floating-point
division operations with a FLOAT_IS_ZERO check e.g.

if (FLOAT_IS_ZERO (sa))
	f = 1.0f;
else
	f = CLAMP (da / sa);

With -fno-trapping-math, the compiler assumes that division will never trap, so it may
re-order the division and the guard and execute the division first.  In most cases,
this would not be an issue, because floating-point exceptions are ignored.  However,
these tests call enable_divbyzero_exceptions() which causes the SIGFPE signal to
be sent to the program when a divide by zero exception is raised.

[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
[2] https://clang.llvm.org/docs/UsersManual.html#controlling-floating-point-behavior

meson.build

@@ -37,6 +37,12 @@ add_project_arguments(
     '-fno-strict-aliasing',
     '-fvisibility=hidden',
     '-Wundef',
+    # -ftrapping-math is the default for gcc, but -fno-trapping-math is the
+    # default for clang.  The FLOAT_IS_ZERO macro is used to guard against
+    # floating-point exceptions, however with -fno-trapping-math, the compiler
+    # can reorder floating-point operations so that they occur before the guard.
+    # Note, this function is ignored in clang < 10.0.0.
+    '-ftrapping-math'
   ]),
   language : ['c']
 )