Add custom beam cursor

In issue jwilm/asacritty#31 a few people complained about Beam cursor
being weird with their fonts, so to make all cursors uniform in
alacritty, a custom cursor has been added for the Beam too.

The beam cursor's height is always the full height of the monospace font
glyph bounding box. The width depends on the font size. It is calculated
using the width of the cell and dividing it by two. The block is always
aligned at the far-left of the cell.

The implementation is identical to the one of the underline cursor, but
it has been refactored so the glyphs are created in `lib.rs`, which can
be used by darwin/macos to implement these features too.

A small bug in the underline cursor has also been fixed, until now the
width was just using the width of the unicode character. Now it should
be using the full width of the monospace glyph bounding box with every
font.

Here are some screenshots for the Beam cursor:
![Small](https://u.teknik.io/v1QH3.png)
![Normal](https://u.teknik.io/RTlp2.png)
![Big](https://u.teknik.io/DLu2M.png)
![Huge](https://u.teknik.io/pSAFX.png)

1 files changed, 42 insertions, 0 deletions

diff --git a/font/src/lib.rs b/font/src/lib.rs
index 5d35e47a..776127ea 100644
--- a/font/src/lib.rs
+++ b/font/src/lib.rs
@@ -65,6 +65,13 @@ pub const UNDERLINE_CURSOR_CHAR: char = '\u{10a3e2}';
 #[cfg(target_os = "macos")]
 pub const UNDERLINE_CURSOR_CHAR: char = '▁';
 
+/// Character used for the beam cursor
+#[cfg(not(target_os = "macos"))]
+// This is part of the private use area and should not conflict with any font
+pub const BEAM_CURSOR_CHAR: char = '\u{10a3e3}';
+#[cfg(target_os = "macos")]
+pub const BEAM_CURSOR_CHAR: char = '▎';
+
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct FontDesc {
     name: String,
@@ -209,6 +216,41 @@ impl Default for RasterizedGlyph {
     }
 }
 
+// Returns a custom underline cursor character
+pub fn get_underline_cursor_glyph(descent: i32, width: i32) -> Result<RasterizedGlyph, Error> {
+    // Create a new rectangle, the height is half the distance between
+    // bounding box bottom and the baseline
+    let height = i32::abs(descent / 2);
+    let buf = vec![255u8; (width * height * 3) as usize];
+
+    // Create a custom glyph with the rectangle data attached to it
+    return Ok(RasterizedGlyph {
+        c: UNDERLINE_CURSOR_CHAR,
+        top: descent + height,
+        left: 0,
+        height,
+        width,
+        buf: buf,
+    });
+}
+
+// Returns a custom beam cursor character
+pub fn get_beam_cursor_glyph(ascent: i32, height: i32, width: i32) -> Result<RasterizedGlyph, Error> {
+    // Create a new rectangle
+    let beam_width = (f64::from(width) / 5.) as i32;
+    let buf = vec![255u8; (beam_width * height * 3) as usize];
+
+    // Create a custom glyph with the rectangle data attached to it
+    return Ok(RasterizedGlyph {
+        c: BEAM_CURSOR_CHAR,
+        top: ascent,
+        left: 0,
+        height,
+        width: beam_width,
+        buf: buf,
+    });
+}
+
 struct BufDebugger<'a>(&'a [u8]);
 
 impl<'a> fmt::Debug for BufDebugger<'a> {