Fix consecutive separate ligatures being rendered in the same group

Rendering multiple ligatures in the smae group is bad for performance
and also causes incorrect rendering if the last ligature in the group
does not fit into the number of cells in the group.

kitty/core_text.m

@@ -200,6 +200,15 @@ glyph_id_for_codepoint(PyObject *s, char_type ch) {
     return glyphs[0];
 }
 
+bool
+is_glyph_empty(PyObject *s, glyph_index g) {
+    CTFace *self = (CTFace*)s;
+    CGGlyph gg = g;
+    CGRect bounds;
+    CTFontGetBoundingRectsForGlyphs(self->ct_font, kCTFontOrientationHorizontal, &gg, &bounds, 1);
+    return bounds.size.width <= 0;
+}
+
 static inline float
 scaled_point_sz() {
     return ((global_state.logical_dpi_x + global_state.logical_dpi_y) / 144.0) * global_state.font_sz_in_pts;