clippy: Add safety documentation and clean up unsafe methods (#33748)

This change:

1. Adds safety documentation where it was missing.
2. Limits the scope of unsafe code in some cases to where it is actually
   unsafe.
3. Converts some free functions to associated functions and methods,
   thereby making them more likely to be called safely.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/canvas/canvas_data.rs

@@ -656,7 +656,7 @@ impl<'a> CanvasData<'a> {
             ideographic_baseline,
             alphabetic_baseline,
             hanging_baseline,
-        } = match font.get_baseline() {
+        } = match font.baseline() {
             Some(baseline) => baseline,
             None => FontBaseline {
                 hanging_baseline: ascent * HANGING_BASELINE_DEFAULT,