Previously the private APIs for QTriangulator and QTriangulatingStroker
were located in src/gui/opengl because they were used by the OpenGL
paint engine. These API's are not actually specific to OpenGL however,
and were not being built when QT_NO_OPENGL was defined. It makes more
sense for these classes to belong in the painting subgroup.
Aside from the OpenGL paint engine, these private APIs are used by
QtLocation to triangulate polylines to be rendered by QtQuick.
Reviewed-by: Laszlo Agocs <firstname.lastname@example.org>