swf: Add Twips::HALF

Twips::HALF represents 10 twips, i.e. 0.5 pixels.
This constant is useful when calculating exact positions of lines
to be drawn, as they are often drawn on whole pixels.

swf/src/types/twips.rs

@@ -40,6 +40,15 @@ impl Twips {
     /// ```
     pub const ONE: Self = Self(Self::TWIPS_PER_PIXEL);
 
+    /// The `Twips` object with a value of `0.5` pixels.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// assert_eq!(swf::Twips::HALF.to_pixels(), 0.5);
+    /// ```
+    pub const HALF: Self = Self(Self::TWIPS_PER_PIXEL / 2);
+
     /// Creates a new `Twips` object. Note that the `twips` value is in twips,
     /// not pixels. Use the [`from_pixels`] method to convert from pixel units.
     ///