Project simple Qt types (QRect/F, QSize/F, QRgba64) as plain Go structs

[mappu]

In Qt C++ these POCO-like types are normally stack-allocated and passed by value reference. But MIQT currently has to move them to heap pointers to cross the C ABI, requiring extra CGO roundtrips to allocate them from the Qt side.

By using plain Go structs instead, they can be better stored on the stack.