config: add tweak.box-drawing-solid-shades=yes|no

When enabled, shades are rendered as solid blocks, using a darker
variant of the current foreground color.

When disabled, shades are instead rendered in a checker box pattern,
using the foreground color unmodified.

Default is enabled.

config.h

@@ -229,6 +229,7 @@ struct config {
         uint64_t delayed_render_upper_ns;
         off_t max_shm_pool_size;
         float box_drawing_base_thickness;
+        bool box_drawing_solid_shades;
     } tweak;
 
     user_notifications_t notifications;