Fix centering of various 3D trees

[Terkwood]

issue

3D trees aren't centered ; hack the position somehow

sample code

See https://github.com/Terkwood/png-find-color

planing

https://docs.rs/png/0.16.8/png/struct.Reader.html#method.next_row and https://docs.rs/png/0.16.8/png/struct.Reader.html#method.output_line_size

https://godotengine.org/qa/18955/image-get_pixel-always-returns-zero?show=18968#a18968