`surfaceToPolygon` is only useful for `QgsCurvePolygon` which inherits
from `QgsSurface`. The next commit will introduce
`QgsPolyhedralSurface` which also inherits from `QgsSurface`. For
`QgsPolyhedralSurface`, it does not make sense to have
`surfaceToPolygon`.
By moving `surfaceToPolygon` definition in `QgsCurvePolygon` this
allows to keep the functionality.
For non-point geometry subclasses (points are always valid!) we
now cache the results of a geometry validity check. Subsequent
checks utilise the cached result wherever possible.
Because QgsGeometry/QgsFeature objects are implicitly shared, this
means that we avoid a *lot* of duplicate validity checks as
features and geometries are thrown around during processing model
execution.
