Improvements to oled.centre_text()

[roryjamesallen]

Reviewing #328 I noticed some improvements that should have been made long ago to centre_text to allow it to work with larger displays where the 3 line rule is not applicable, and with other character sizes where some numbers were still hard coded for 8x8

[mjaskula]

This reminds me that we also have an implementation of centre_text() in experimental/custom_font.py. I don't think that it should influence the changes in this PR, but we should be aware of it when we finally resolve the three implementations.

[mjaskula]

These changes make sense to me.

[roryjamesallen]

This reminds me that we also have an implementation of centre_text() in experimental/custom_font.py. I don't think that it should influence the changes in this PR, but we should be aware of it when we finally resolve the three implementations.

Good point! I looked over that implementation and there isn't anything crucial that's different to the changes in this PR, I think for the X it'll be a bigger overhaul anyway given that larger fonts are more likely to be used on the larger display, so more consideration is probably gonna be needed when it comes to it