a cell is empty only if it also has no styling

Previously we only checked if it had no character. With text shaping,
this was causing runs of only empty backgrounds to not render.

src/Grid.zig

@@ -579,8 +579,8 @@ pub fn updateCell(
         });
     }
 
-    // If the cell is empty then we draw nothing in the box.
+    // If the cell has a character, draw it
-    if (!cell.empty()) {
+    if (cell.char > 0) {
         // Render
         const face = self.font_group.group.faceFromIndex(shaper_run.font_index);
         const glyph = try self.font_group.renderGlyph(

src/font/Shaper.zig

@@ -276,6 +276,42 @@ test "run iterator" {
     }
 }
 
+test "run iterator: empty cells with background set" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var testdata = try testShaper(alloc);
+    defer testdata.deinit();
+
+    {
+        // Make a screen with some data
+        var screen = try terminal.Screen.init(alloc, 3, 5, 0);
+        defer screen.deinit();
+        try screen.testWriteString("A");
+
+        // Get our first row
+        const row = screen.getRow(.{ .active = 0 });
+        row.getCellPtr(1).bg = try terminal.color.Name.cyan.default();
+        row.getCellPtr(1).attrs.has_bg = true;
+        row.getCellPtr(2).fg = try terminal.color.Name.yellow.default();
+        row.getCellPtr(2).attrs.has_fg = true;
+
+        // Get our run iterator
+        var shaper = testdata.shaper;
+        var it = shaper.runIterator(testdata.cache, screen.getRow(.{ .screen = 0 }));
+        var count: usize = 0;
+        while (try it.next(alloc)) |run| {
+            count += 1;
+
+            // The run should have length 3 because of the two background
+            // cells.
+            try testing.expectEqual(@as(u32, 3), shaper.hb_buf.getLength());
+            const cells = try shaper.shape(run);
+            try testing.expectEqual(@as(usize, 3), cells.len);
+        }
+        try testing.expectEqual(@as(usize, 1), count);
+    }
+}
 test "shape" {
     const testing = std.testing;
     const alloc = testing.allocator;

src/terminal/Screen.zig

@@ -183,7 +183,14 @@ pub const Cell = struct {
 
     /// True if the cell should be skipped for drawing
     pub fn empty(self: Cell) bool {
-        return self.char == 0;
+        // Get our backing integer for our packed struct of attributes
+        const AttrInt = @Type(.{ .Int = .{
+            .signedness = .unsigned,
+            .bits = @bitSizeOf(@TypeOf(self.attrs)),
+        } });
+
+        // We're empty if we have no char AND we have no styling
+        return self.char == 0 and @bitCast(AttrInt, self.attrs) == 0;
     }
 
     /// The width of the cell.