UI/nvim_ui_attach(): add `override` option

Before now, Nvim always degrades UI capabilities to the lowest-common
denominator. For example, if any connected UI has `ext_messages=false`
then `ext_messages=true` requested by any other connected UI is ignored.

Now `nvim_ui_attach()` supports `override=true`, which flips the
behavior: if any UI requests an `ext_*` UI capability then the
capability is enabled (and the legacy behavior is disabled).

Legacy UIs will be broken while a `override=true` UI is connected, but
it's useful for debugging: you can type into the TUI and observe the UI
events from another connected (UI) client. And the legacy UI will
"recover" after the `override=true` UI disconnects.

Example using pynvim:

    >>> n.ui_attach(2048, 2048, rgb=True, override=True, ext_multigrid=True, ext_messages=True, ext_popupmenu=True)
    >>> while True: n.next_message();

1 files changed, 10 insertions, 0 deletions

diff --git a/src/nvim/api/ui.c b/src/nvim/api/ui.c
index fd94418d48..9f12ae6888 100644
--- a/src/nvim/api/ui.c
+++ b/src/nvim/api/ui.c
@@ -110,6 +110,7 @@ void nvim_ui_attach(uint64_t channel_id, Integer width, Integer height,
   ui->width = (int)width;
   ui->height = (int)height;
   ui->rgb = true;
+  ui->override = false;
   ui->grid_resize = remote_ui_grid_resize;
   ui->grid_clear = remote_ui_grid_clear;
   ui->grid_cursor_goto = remote_ui_grid_cursor_goto;
@@ -236,6 +237,15 @@ void nvim_ui_set_option(uint64_t channel_id, String name,
 static void ui_set_option(UI *ui, bool init, String name, Object value,
                           Error *error)
 {
+  if (strequal(name.data, "override")) {
+    if (value.type != kObjectTypeBoolean) {
+      api_set_error(error, kErrorTypeValidation, "override must be a Boolean");
+      return;
+    }
+    ui->override = value.data.boolean;
+    return;
+  }
+
   if (strequal(name.data, "rgb")) {
     if (value.type != kObjectTypeBoolean) {
       api_set_error(error, kErrorTypeValidation, "rgb must be a Boolean");