Empty QgsCoordinateTransformContext in function calls

[Withalion]

Description
Currently we are calling QGIS coordinate transform functions with new/empty QgsCoordinateTransformContext, which is frowned upon by QGIS itself. It might introduce some issue when users setup custom transforms in QGIS as mentioned by @wonder-sk in here

We should use current QgsCoordinateTransformContext from active project.