The colors/fonts preference page uses images for the items shown in a
tree viewer that preview the selected color. The current implementation
has two limitations:
- It does not properly scale with the current monitor zoom
- It uses a wrong image size based on the tree item height instead of
the tree item image size, such that the rendered image is scaled and may
loose information

This change removes all custom extraction of the image size as the image
simply needs to have the same size as all other images default to
(16x16) to fit. It also wraps the implementation info an ImageGcDrawer
to be monitor-scale-aware.