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/config.h

@@ -349,6 +349,7 @@ struct sway_config {
 	enum sway_container_layout default_layout;
 	char *font;
 	size_t font_height;
+	size_t font_baseline;
 	bool pango_markup;
 	size_t urgent_timeout;
 	enum sway_fowa focus_on_window_activation;