Add option for box-drawing characters to scale with font zoom

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This is my temporary patch workaround (0.48.2) but it's certainly too dirty (hardcoded constants, etc.)
I might get around to a better version once I figure out how to add the rest of the options
stuff/docs to make it cleaner.
--- a/kitty/fonts.c
+++ b/kitty/fonts.c
@@ -1069,7 +1069,11 @@ render_box_cell(FontGroup *fg, RunFont rf, CPUCell *cpu_cell, GPUCell *gpu_cell,
         return;
     }
     FontCellMetrics unscaled_metrics = fg->fcm;
+    // apply_scale_to_font_group() overwrites fg->font_sz_in_pts with scaled size
+    const double base_font_size = fg->font_sz_in_pts;
     float scale = apply_scale_to_font_group(fg, &rf);
+    // 11.0 is the default font size
+    const double box_scale = scale * base_font_size / 11.0;
     ensure_canvas_can_fit(fg, num_glyphs + 1, rf.scale);
     FontCellMetrics scaled_metrics = fg->fcm;
     if (scale != 1) apply_scale_to_font_group(fg, NULL);
@@ -1087,7 +1091,7 @@ render_box_cell(FontGroup *fg, RunFont rf, CPUCell *cpu_cell, GPUCell *gpu_cell,
     for (unsigned i = 0, cnum = 0; i < num_glyphs; i++) {
         unsigned int ch = global_glyph_render_scratch.lc->chars[cnum++];
         while (!ch) ch = global_glyph_render_scratch.lc->chars[cnum++];
-        render_box_char(ch, fg->canvas.alpha_mask, src.right, src.bottom, fg->logical_dpi_x, fg->logical_dpi_y, scale);
+        render_box_char(ch, fg->canvas.alpha_mask, src.right, src.bottom, fg->logical_dpi_x, fg->logical_dpi_y, box_scale);
         dest.left = i * scaled_metrics.cell_width + right_shift; dest.right = dest.left + scaled_metrics.cell_width;
         render_alpha_mask(fg->canvas.alpha_mask, fg->canvas.buf, &src, &dest, src.right, mask_stride, 0xffffff);
     }