renderer/metal: reset screen dirty flags

I introduced screen dirty flags fairly late, and never reset them. As a
result, dirty tracking basically gets disabled as soon as selection
changes in any form. This happens frequently because _unsetting_
selection also resets selection which happens for various events.

src/renderer/Metal.zig

@@ -932,6 +932,7 @@ pub fn updateFrame(
         // success and reset while we hold the lock. This is much easier
         // than coordinating row by row or as changes are persisted.
         state.terminal.flags.dirty = .{};
+        state.terminal.screen.dirty = .{};
         {
             var it = state.terminal.screen.pages.pageIterator(
                 .right_down,