render: optionally enable heuristics that deal with private usage area chars

Try to detect double-width *glyphs* for single-width *characters*, and
allow them to overflow into the next cell.

This is only done for single-width chars with a glyph width that is at
least 1.5 cells wide, but at most 3 cells.

The feature is gated by the new
‘tweak.allow-overflowing-double-width-glyphs’, and is disabled by
default.

Closes #116

config.h

@@ -144,6 +144,7 @@ struct config {
     bool hold_at_exit;
 
     struct {
+        bool allow_overflowing_double_width_glyphs;
         bool render_timer_osd;
         bool render_timer_log;
         uint64_t delayed_render_lower_ns;