Add support for doing a (0.0, 0.0, 0.0, 0.0) initialization in the default constructor of fvec4SIMD().

This is disabled by default, but enabled with GLM_SIMD_ENABLE_DEFAULT_INIT. This makes toggling between SIMD and non-SIMD builds a bit easier.

glm/gtx/simd_vec4.inl

@@ -20,6 +20,9 @@ struct mask
 // Implicit basic constructors
 
 GLM_FUNC_QUALIFIER fvec4SIMD::fvec4SIMD()
+#ifdef GLM_SIMD_ENABLE_DEFAULT_INIT
+    : Data(_mm_set_ps(0.0f, 0.0f, 0.0f, 0.0f))
+#endif
 {}
 
 GLM_FUNC_QUALIFIER fvec4SIMD::fvec4SIMD(__m128 const & Data) :