Align titles to baseline

This does the following:

* Adds a baseline argument to get_text_size (the baseline is the
distance from the top of the texture to the baseline).
* Stores the baseline in the container when calculating the title
height.
* Takes the baseline into account when calculating the config's max font
height.
* When rendering, pads the textures according to the baseline so they
line up.

include/sway/tree/container.h

@@ -117,6 +117,7 @@ struct sway_container {
 	struct wlr_texture *title_unfocused;
 	struct wlr_texture *title_urgent;
 	size_t title_height;
+	size_t title_baseline;
 
 	struct {
 		struct wl_signal destroy;