input: restore ability to combine mouse buttons

Key bindings are decided on the "down" event, so if the prefix is not
unique, the first/shortest will be used (e.g. when both "a" and "a-b"
are mapped, "a" will always be chosen).

This also breaks combining multiple mouse buttons. But it seems users
expect it to work, and it's indeed a bit strange that it shouldn't work,
as mouse bindings are emitted on the key "up" event, not "down" (if the
shorter binding didn't emit a command yet, why shouldn't it be
combinable).

Deal with this by clearing the key history when a command is actually
emitted, instead of when a command is decided. This means if both
MOUSE_BTN0 and MOUSE_BTN0-MOUSE_BTN1 are mapped, the sequence of holding
down BTN0 and then BTN1 will redecide the current command. On the other
hand, if BTN0 is released before BTN1 is pressed, the command is
emitted, and the key history is deleted. So the BTN1 press will not
trigger BTN0-BTN1.

For normal keys, nothing should change, because commands are emitted on
the "down" event already, so the key history is always cleared.

Might fix #902.

CC: @mpv-player/stable (if this fix is successful)

input/input.c

@@ -557,6 +557,7 @@ static void release_down_cmd(struct input_ctx *ictx, bool drop_current)
     if (!drop_current && ictx->current_down_cmd &&
         ictx->current_down_cmd->key_up_follows)
     {
+        memset(ictx->key_history, 0, sizeof(ictx->key_history));
         ictx->current_down_cmd->key_up_follows = false;
         queue_add_tail(&ictx->cmd_queue, ictx->current_down_cmd);
         ictx->got_new_events = true;
@@ -596,15 +597,10 @@ static struct mp_cmd *resolve_key(struct input_ctx *ictx, int code)
 {
     update_mouse_section(ictx);
     struct mp_cmd *cmd = get_cmd_from_keys(ictx, NULL, code);
-    if (cmd && cmd->id != MP_CMD_IGNORE) {
-        memset(ictx->key_history, 0, sizeof(ictx->key_history));
-        if (!should_drop_cmd(ictx, cmd))
-            return cmd;
-        talloc_free(cmd);
-        return NULL;
-    }
-    talloc_free(cmd);
     key_buf_add(ictx->key_history, code);
+    if (cmd && cmd->id != MP_CMD_IGNORE && !should_drop_cmd(ictx, cmd))
+        return cmd;
+    talloc_free(cmd);
     return NULL;
 }
 
@@ -673,6 +669,8 @@ static void interpret_key(struct input_ctx *ictx, int code, double scale)
         return;
     }
 
+    memset(ictx->key_history, 0, sizeof(ictx->key_history));
+
     cmd->scale = scale;
 
     if (cmd->key_up_follows)