Support combining characters. (#121)

Modify the ftxui::Pixel. Instead of storing a wchar, store a
std::wstring. Now a single pixel can store multiple codepoints.
If a codepoint is of size <=0, it will be appended to the previous
pixel.

Only ftxui::text() is supported. ftxui::vtext support still needs to be
added.

This causes the following CPU and memory regression:
- Memory: Pixel size increases by 200% (16 byte => 48byte).
- CPU:    Draw/Second decrease by 62.5% (16k draw/s => 6k draw/s on 80x80)

Both regressions are acceptable. There are still two orders of magnitude
(100x) before the levels where performance/memory concerns begins.

This fixes: https://github.com/ArthurSonzogni/FTXUI/issues/109

src/ftxui/dom/text.cpp

@@ -29,10 +29,15 @@ class Text : public Node {
     if (y > box_.y_max)
       return;
     for (wchar_t c : text_) {
-      if (x > box_.x_max)
-        return;
-      screen.at(x, y) = c;
-      x += wchar_width(c);
+      const int width = wchar_width(c);
+      if (width >= 1) {
+        if (x > box_.x_max)
+          return;
+        screen.PixelAt(x, y).character = c;
+      } else {
+        screen.PixelAt(x - 1, y).character += c;
+      }
+      x += std::max(width, 0);
     }
   }