Fix copyTo on empty fixed-type matrices (#28343)

PR #27972 added _dst.create(size(), type()) in copyTo's empty() block.
In Debug builds, Mat::release() was resetting flags to MAGIC_VAL,
clearing the type information and causing assertion failures when
destination has fixedType().

Preserve type flags in Mat::release() debug mode by using:
  flags = (flags & CV_MAT_TYPE_MASK) | MAGIC_VAL

Thanks to @akretz for suggesting this better approach.

modules/core/src/matrix.cpp

@@ -553,7 +553,7 @@ void Mat::release()
     for(int i = 0; i < dims; i++)
         size.p[i] = 0;
 #ifdef _DEBUG
-    flags = MAGIC_VAL;
+    flags = (flags & CV_MAT_TYPE_MASK) | MAGIC_VAL;
     dims = rows = cols = 0;
     if(step.p != step.buf)
     {