LibGfx: Make Gfx::FontMetrics include the advance of '0' instead of 'M'

CSS actually wants the advance of the ASCII '0' character for its "ch"
units, so let's include that instead of the arbitrarily chosen 'M'.

Userland/Libraries/LibGfx/BitmapFont.cpp

@@ -377,7 +377,7 @@ FontMetrics Font::metrics() const
     return FontMetrics {
         .size = (float)presentation_size(),
         .x_height = (float)x_height(),
-        .glyph_width = (float)glyph_width('M'),
+        .advance_of_ascii_zero = (float)glyph_width('0'),
         .glyph_spacing = (float)glyph_spacing(),
     };
 }

Userland/Libraries/LibGfx/Font.h

@@ -89,7 +89,7 @@ private:
 struct FontMetrics {
     float size { 0 };
     float x_height { 0 };
-    float glyph_width { 0 };
+    float advance_of_ascii_zero { 0 };
     float glyph_spacing { 0 };
 };
 

Userland/Libraries/LibWeb/CSS/Length.cpp

@@ -76,7 +76,7 @@ float Length::relative_length_to_px(Gfx::IntRect const& viewport_rect, Gfx::Font
         return m_value * font_size;
     case Type::Ch:
         // FIXME: Use layout_node.font().glyph_height() when writing-mode is not horizontal-tb (it has to be implemented first)
-        return m_value * (font_metrics.glyph_width + font_metrics.glyph_spacing);
+        return m_value * (font_metrics.advance_of_ascii_zero + font_metrics.glyph_spacing);
     case Type::Rem:
         return m_value * root_font_size;
     case Type::Vw: