CharacterPreview: Get pos after loading sheet

Otherwise the code will break, since the y-position is calculated
based on the height of the character sprite.
CharacterPreview: Fix call to transform.flip()

Apparently you can't specify the names of the parameters, for whatever
reason.
CharacterPreview: Fix shadow rendering

center_y within render_shadow() is unneeded, as pygame.draw.ellipse()
automatically draws from the top-left corner, rather than from the
center of the ellipse as I had previously assumed.
CharacterPreview: Fix Surfaces' transparency
CharacterPreview: Write update()
CharacterPreview: Turn ground_y into a constant

Since the ground will always be in the same position regardless of the
character, there's no point in needing to pass it during
initialization.
CharacterPreview:Record name_font as an attribute

It will be needed after initialization when the character is changed.
CharacterPreview: Write change_character()
CharacterPreview: Write move()
CharacterPreview: Write place_offscreen()
CharacterPreview + is_onscreen(), is_onscreen()