Fix RichTextLabel character horizontal offset calculation

[Listwon]

This PR is not relevant to master branch, because on master the x offset in RichTextLabel is properly treated as float (just like in Label code on 3.x branch).
Fixes #22580

Horizontal offsets were assigned to the integer variables while the actual type of value returned by get_char_size is float (and is properly handled in plain Label). We were losing data in cases where character sizes were not integer values.

[Listwon]

There is still one problem which I'm stuggling to solve, here you can see one character not being moved to the next line:

MRP: render_text_bug.zip

[Listwon]

The last problem I mentioned is not an issue connected directly with offsets, but simply with major differences how line/word wrapping is handled in 3.x RichTextLabel (it was reworked on master branch). So from my perspecrive this PR does its purpose - fixes the offsets so the characters are not tightened and unreadable on scaled viewports. Changing the wrapping code is the topic for another PR.

[akien-mga]

Thanks!