Optimize VNC screen update to reduce delays

Previously, the VNC renderer fed the damaged region to the buffer line
by line, resulting in significant rendering delays due to frequent
buffer updates.

The implementation has been optimized to aggregate all updates during
the current screen refresh cycle. The buffer is now updated once per
refresh with the complete damaged region, reducing rendering delays and
preventing screen tearing.

Close sysprog21#81

backend/vnc.c

@@ -75,16 +75,8 @@ static void _twin_vnc_put_span(twin_coord_t left,
     size_t span_width = right - left;
 
     memcpy(fb_pixels, pixels, span_width * sizeof(*fb_pixels));
-
-    pixman_region_init_rect(&tx->damage_region, left, top, span_width, 1);
-
-    if (pixman_region_not_empty(&tx->damage_region)) {
-        nvnc_display_feed_buffer(tx->display, tx->current_fb,
-                                 &tx->damage_region);
-        pixman_region_clear(&tx->damage_region);
-    }
-    aml_poll(tx->aml, 0);
-    aml_dispatch(tx->aml);
+    pixman_region_union_rect(&tx->damage_region, &tx->damage_region, left, top,
+                             span_width, 1);
 }
 
 static void twin_vnc_get_screen_size(twin_vnc_t *tx, int *width, int *height)
@@ -96,9 +88,13 @@ static void twin_vnc_get_screen_size(twin_vnc_t *tx, int *width, int *height)
 static bool _twin_vnc_work(void *closure)
 {
     twin_screen_t *screen = SCREEN(closure);
-
-    if (twin_screen_damaged(screen))
+    twin_vnc_t *tx = PRIV(closure);
+    if (twin_screen_damaged(screen)) {
+        pixman_region_clear(&tx->damage_region);
         twin_screen_update(screen);
+        nvnc_display_feed_buffer(tx->display, tx->current_fb,
+                                 &tx->damage_region);
+    }
     return true;
 }
 
@@ -262,6 +258,14 @@ twin_context_t *twin_vnc_init(int width, int height)
     return NULL;
 }
 
+static bool twin_vnc_poll(twin_context_t *ctx)
+{
+    twin_vnc_t *tx = PRIV(ctx);
+    aml_poll(tx->aml, 0);
+    aml_dispatch(tx->aml);
+    return true;
+}
+
 static void twin_vnc_configure(twin_context_t *ctx)
 {
     int width, height;
@@ -288,6 +292,7 @@ static void twin_vnc_exit(twin_context_t *ctx)
 
 const twin_backend_t g_twin_backend = {
     .init = twin_vnc_init,
+    .poll = twin_vnc_poll,
     .configure = twin_vnc_configure,
     .exit = twin_vnc_exit,
 };