Introduce VariantImplicitConvert<> template for types that can be implicitly converted to and from Variant.

De-duplicate a lot of `VariantGetInternalPtr`, `VariantInternalAccessor`, `VariantInitializer` and `VariantDefaultInitializer`.

Author: Ivorforce

core/variant/variant.h

@@ -171,6 +171,12 @@ class Variant {
 
 	friend struct _VariantCall;
 	friend class VariantInternal;
+	template <typename>
+	friend struct _VariantGetInternalPtrLocal;
+	template <typename>
+	friend struct _VariantGetInternalPtrElsewhere;
+	template <typename>
+	friend struct _VariantGetInternalPtrPackedArrayRef;
 	// Variant takes 24 bytes when real_t is float, and 40 bytes if double.
 	// It only allocates extra memory for AABB/Transform2D (24, 48 if double),
 	// Basis/Transform3D (48, 96 if double), Projection (64, 128 if double),

core/variant/variant_construct.cpp

@@ -125,8 +125,8 @@ void Variant::_register_variant_constructors() {
 
 	add_constructor<VariantConstructNoArgs<Transform2D>>(sarray());
 	add_constructor<VariantConstructor<Transform2D, Transform2D>>(sarray("from"));
-	add_constructor<VariantConstructor<Transform2D, float, Vector2>>(sarray("rotation", "position"));
-	add_constructor<VariantConstructor<Transform2D, float, Size2, float, Vector2>>(sarray("rotation", "scale", "skew", "position"));
+	add_constructor<VariantConstructor<Transform2D, double, Vector2>>(sarray("rotation", "position"));
+	add_constructor<VariantConstructor<Transform2D, double, Size2, double, Vector2>>(sarray("rotation", "scale", "skew", "position"));
 	add_constructor<VariantConstructor<Transform2D, Vector2, Vector2, Vector2>>(sarray("x_axis", "y_axis", "origin"));
 
 	add_constructor<VariantConstructNoArgs<Plane>>(sarray());