Deprecate QGenericMatrix<>

According to the email discussion, we should deprecate QGenericMatrix<> template class. However, if we deprecate it, we also deprecate all its typedefs, like QMatrix2x2, QMatrix3x3 and so on.

So issues to be solved:

1. One of the most used typedefs of QGenericMatrix is QMatrix3x3. That's why the proposition was to add QMatrix3x3 class (as a simple wrapper around float[3][3]). However, we can't add this class to Qt 5.15, since we will still keep the obsoleted QMatrix3x3 wrapper. So, one of the solutions would be to deprecate QGenericMatrix<> and all its typedefs but QMatrix3x3. Later, in Qt 6 we get rid of QGenericMatrix<> and all its typedefs (including QMatrix3x3) and provide QMatrix3x3 wrapper class around float[3][3] with the same API that QGenericMatrix<> has currently. In this way we should keep the source compatibility. Would that work?

Another option would be to add a 3x3 matrix class in Qt 5.15 under some other name, but I think we don't want that.

Other options?

2. Other typedefs, like QMatrix2x2, QMatrix2x3, etc... are used in public Qt API:

QOpenGLShaderProgram::setUniformValue(int location, const QMatrix2x2& value);
QOpenGLShaderProgram::setUniformValueArray(int location, const QMatrix3x4 *values, int count);

...etc. What to do with them? Just deprecate?

3. We have some usages of QMatrix2x2, QMatrix2x3 typedefs inside Qt3D module, in:

qt3d/src/plugins/sceneparsers/gltf/gltfimporter.cpp
qt3d/src/plugins/sceneparsers/gltf/gltfio.cpp
qt3d/src/plugins/sceneparsers/gltfexport/gltfexporter.cpp

What shall we do there?