terminal: break out scaling factor updating, and reduce number of calls to render_resize()

Break out the logic that updates the terminal’s scaling factor value,
from render_resize(), to a new function, term_update_scale(). This
allows us to update the scaling factor without a full grid resize.

We also change how we pick the scaling factor (when fractional scaling
is not in use). Before, we’d use the highest scaling factor from all
monitors we were mapped on. Now, we use the scaling factor from the
monitor we were *last* mapped on.

Then, add a boolean parameter to term_set_fonts(), and when
false, *don’t* call render_resize_force().

Also change term_font_dpi_changed() to only return true if the font
was changed in any way.

Finally, rewrite update_term_for_output_change() to:

* Call term_update_scale() before doing anything else
* Call render_resize{,_force} *last*, and *only* if either the scale
  or the fonts were updated.

This fixes several things:

* A bug where we failed to update the fonts when fractional scaling
  was in use, and we guessed the initial scale/DPI wrong. The bug
  happened because updated the internal "preferred" scale value, and a
  later call to render_resize() updated the terminal’s scale value,
  but since that code path didn’t call term_font_dpi_changed() (and it
  shouldn’t), the fonts weren’t resized properly.

* It ensures we only resize the grid *once* when the scaling factor,
  or DPI is changed. Before this, we’d resize it twice. And this
  happened when e.g. dragging the window between monitors.

terminal.h

@@ -736,6 +736,7 @@ bool term_to_slave(struct terminal *term, const void *data, size_t len);
 bool term_paste_data_to_slave(
     struct terminal *term, const void *data, size_t len);
 
+bool term_update_scale(struct terminal *term);
 bool term_font_size_increase(struct terminal *term);
 bool term_font_size_decrease(struct terminal *term);
 bool term_font_size_reset(struct terminal *term);