feat(backend): add CellSized trait for querying cell dimensions in physical and CSS pixels

[junkdog]

enabler for properly handling mouse wheel scroll events in #142

revolves around the CellSized trait, implemented by all backends.

pub trait CellSized {
    /// Returns the size of a cell in physical (device) pixels as `(width, height)`.
    fn cell_size_px(&self) -> (f32, f32);

    /// Returns the size of a cell in CSS (logical) pixels as `(width, height)`.
    fn cell_size_css_px(&self) -> (f32, f32);
}

[junkdog]

hmm, tested with the "minimal" example: the offset bug when zooming in the webgl2 backend has returned... it's unrelated to these changes, as it exhibits the same buggy behavior one main. i'll fix it separately (+ add some form of integration test...)

[junkdog]

the offset bug when zooming in the webgl2 backend has returned

fixed it in beamterm - which also translated to simplified mouse handling in ratzilla (-35 loc)

[orhun]

Cool, is this ready for review?

[junkdog]

Cool, is this ready for review?

it is now; and the offset bug was fixed after merging main into the branch.

[orhun]

LGTM!

[orhun]

I feel like we should include this trait in either util module or move it under backend