Fix column vs row-major confusion in QMatrix4x4 API

QMatrix4x4 documentation and API says or implies row-major order but the data() and constData() functions return the data in column major order, and also the internal storage is in column major order. The doco for data() and constData() are silent on the format the data is returned in, so users assume it is row-major.

This is not a bug in functionality as such - we don't count on the result from the data() or constData() functions as being in row-major order anywhere.

The reason is that those functions are used for passing the data to OpenGL which expects column-major order.

The sorts of confusion users might have is that:

qreal *mat_data = { .... };   // some data

QMatrix4x4 m1(mat_data);
QMatrix4x4 m2(m1.data);

m1 != m2;  

This is where constructors fall miserably short.

QMatrix4x4::fromColumnOrderData(...)
QMatrix4x4::fromRowOrderData(...) 

Would be so much less ambigous.

The constructor is row-order because it is the natural way to write the array data.

data = {
   m11, m12, m13, m14,
   m21, m22, m23, m24,
   ...
}

With column order that you would basically have to specify the transposed matrix, which I tend to find rather confusing. We could work on the naming to make things more consistent though...

If we called the data accessors columnOrderData() and columnOrderConstData(), or a better/shorter name if you have it, and deprecate the data()/constData() functions, it would probably be more self-explanatory. Then deprecate the data constructor for the benefit of a factory function, and the confusion would be gone from code.

Also needs an example showing how to correctly use these functions, eg with a simple translation matrix. Obviously also unit tests.