[Feature Request] Change estimate_text_size to take &mut self

[ecton]

What is the feature ?

I think that DrawingBackend:estimate_text_size should accept an &mut self instead of &self.

(Optional) Why this feature is useful and how people would use the feature ?

I think that it's reasonable to expect that a drawing surface needs to alter its state (current font settings) to be able to perform text measurements.

I say this because while adding plotters support for Kludgine, I ended up switching from implementing DrawingBackend directly on my renderer type to implementing it on a RefCell<&mut Renderer<..>>, because my set_font_family function requires &mut self.

If this function accepted an &mut self, I could avoid a second layer of RefCell -- the first layer being the one DrawingArea utilizes already.