font/sprite: add explicit underline cursor

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

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,
                     };