url-mode: underline URLs using the color from colors.urls

This is implemented by allocating one of the (few!) remaining bits in
the cells’ attribute struct to indicate the cell should be “URL
highlighted”.

render_cell() looks at this bit and draws an underline using the color
from colors.urls (defaults to regular3 - i.e. yellow).

A new function, url_tag_cells(), iterates the currently detected URLs
and sets the new ‘url’ attribute flag on the affected cells.

Note: this is done in a separate function to keep urls_collect() free
from as many dependencies as possible.

urls_reset() is updated to *clear* the ‘url’ flag (and thus implicitly
also results in a grid refresh, _if_ there were any URLs).

We now exit URL mode on *any* client application input. This needs to
be so since we can’t know if the URLs we previously detected are still
valid.

render.c

@@ -616,6 +616,15 @@ render_cell(struct terminal *term, pixman_image_t *pix,
     if (cell->attrs.strikethrough)
         draw_strikeout(term, pix, font, &fg, x, y, cell_cols);
 
+    if (unlikely(cell->attrs.url)) {
+        pixman_color_t url_color = color_hex_to_pixman(
+            term->conf->colors.use_custom.url
+            ? term->conf->colors.url
+            : term->colors.table[3]
+            );
+        draw_underline(term, pix, font, &url_color, x, y, cell_cols);
+    }
+
 draw_cursor:
     if (has_cursor && (term->cursor_style != CURSOR_BLOCK || !term->kbd_focus))
         draw_cursor(term, cell, font, pix, &fg, &bg, x, y, cell_cols);