Defaults the default constructors to make those types trivial / they …

…get passed correctly by interop layer in arm64

dotnet/runtime#106471

imgui.h

@@ -264,7 +264,7 @@ IM_MSVC_RUNTIME_CHECKS_OFF
 struct ImVec2
 {
     float                                   x, y;
-    constexpr ImVec2()                      : x(0.0f), y(0.0f) { }
+    ImVec2() = default;
     constexpr ImVec2(float _x, float _y)    : x(_x), y(_y) { }
     float& operator[] (size_t idx)          { IM_ASSERT(idx == 0 || idx == 1); return ((float*)(void*)(char*)this)[idx]; } // We very rarely use this [] operator, so the assert overhead is fine.
     float  operator[] (size_t idx) const    { IM_ASSERT(idx == 0 || idx == 1); return ((const float*)(const void*)(const char*)this)[idx]; }
@@ -277,7 +277,7 @@ struct ImVec2
 struct ImVec4
 {
     float                                                     x, y, z, w;
-    constexpr ImVec4()                                        : x(0.0f), y(0.0f), z(0.0f), w(0.0f) { }
+    ImVec4() = default;
     constexpr ImVec4(float _x, float _y, float _z, float _w)  : x(_x), y(_y), z(_z), w(_w) { }
 #ifdef IM_VEC4_CLASS_EXTRA
     IM_VEC4_CLASS_EXTRA     // Define additional constructors and implicit cast operators in imconfig.h to convert back and forth between your math types and ImVec4.
@@ -2645,7 +2645,7 @@ struct ImColor
 {
     ImVec4          Value;
 
-    constexpr ImColor()                                             { }
+    ImColor() = default;
     constexpr ImColor(float r, float g, float b, float a = 1.0f)    : Value(r, g, b, a) { }
     constexpr ImColor(const ImVec4& col)                            : Value(col) {}
     constexpr ImColor(int r, int g, int b, int a = 255)             : Value((float)r * (1.0f / 255.0f), (float)g * (1.0f / 255.0f), (float)b * (1.0f / 255.0f), (float)a* (1.0f / 255.0f)) {}