Output: flag gamma as dirty on enable

We can end up with stale gamma settings if we don't re-check the
current gamma settings for the output on enable.

river/Output.zig

@@ -474,6 +474,7 @@ pub fn applyState(output: *Output, state: *wlr.Output.State) error{CommitFailed}
 
 fn handleEnableDisable(output: *Output) void {
     output.updateLockRenderStateOnEnableDisable();
+    output.gamma_dirty = true;
 
     if (output.wlr_output.enabled) {
         // Add the output to root.active_outputs and the output layout if it has not

river/Root.zig

@@ -894,6 +894,7 @@ fn handlePowerManagerSetMode(
     }
 
     output.updateLockRenderStateOnEnableDisable();
+    output.gamma_dirty = true;
 }
 
 fn handleSetGamma(