font/sprite: add explicit underline cursor

Resolves #7651 - uses cursor thickness rather than underline thickness.

src/font/sprite.zig

@@ -32,6 +32,7 @@ pub const Sprite = enum(u32) {
     cursor_rect,
     cursor_hollow_rect,
     cursor_bar,
+    cursor_underline,
 
     test {
         const testing = std.testing;

src/font/sprite/draw/special.zig

@@ -326,3 +326,21 @@ pub fn cursor_bar(
         .height = @intCast(height),
     }, .on);
 }
+
+pub fn cursor_underline(
+    cp: u32,
+    canvas: *font.sprite.Canvas,
+    width: u32,
+    height: u32,
+    metrics: font.Metrics,
+) !void {
+    _ = cp;
+    _ = height;
+
+    canvas.rect(.{
+        .x = 0,
+        .y = @intCast(metrics.underline_position),
+        .width = @intCast(width),
+        .height = @intCast(metrics.cursor_thickness),
+    }, .on);
+}

src/renderer/generic.zig

@@ -3098,7 +3098,7 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                         .block => .cursor_rect,
                         .block_hollow => .cursor_hollow_rect,
                         .bar => .cursor_bar,
-                        .underline => .underline,
+                        .underline => .cursor_underline,
                         .lock => unreachable,
                     };