LibGfx/OpenType: Load x-height metrics from OS/2 table if available

Before this change we always returned the font's point size as the
x-height which was basically never correct.

We now get it from the OS/2 table (if one with version >= 2 is available
in the file). Otherwise we fall back to using the ascent of the 'x'
glyph. Most fonts appear to have a sufficiently modern OS/2 table.

Userland/Libraries/LibGfx/Font/VectorFont.h

@@ -17,6 +17,7 @@ struct ScaledFontMetrics {
     float ascender { 0 };
     float descender { 0 };
     float line_gap { 0 };
+    float x_height { 0 };
 
     float height() const
     {