renderer: fg/bg/cursor change must mark the screen as dirty

Fixes #2372

src/renderer/Metal.zig

@@ -745,6 +745,12 @@ fn displayLinkCallback(
     };
 }
 
+/// Mark the full screen as dirty so that we redraw everything.
+pub fn markDirty(self: *Metal) void {
+    // This is how we force a full rebuild with metal.
+    self.cells_viewport = null;
+}
+
 /// Called when we get an updated display ID for our display link.
 pub fn setMacOSDisplayID(self: *Metal, id: u32) !void {
     if (comptime DisplayLink == void) return;

src/renderer/OpenGL.zig

@@ -645,6 +645,12 @@ pub fn hasVsync(self: *const OpenGL) bool {
     return false;
 }
 
+/// See Metal.
+pub fn markDirty(self: *OpenGL) void {
+    // Do nothing, we don't have dirty tracking yet.
+    _ = self;
+}
+
 /// Callback when the focus changes for the terminal this is rendering.
 ///
 /// Must be called on the render thread.

src/renderer/Thread.zig

@@ -358,14 +358,17 @@ fn drainMailbox(self: *Thread) !void {
 
             .foreground_color => |color| {
                 self.renderer.foreground_color = color;
+                self.renderer.markDirty();
             },
 
             .background_color => |color| {
                 self.renderer.background_color = color;
+                self.renderer.markDirty();
             },
 
             .cursor_color => |color| {
                 self.renderer.cursor_color = color;
+                self.renderer.markDirty();
             },
 
             .resize => |v| {