Cached glyphs used to have a 2px border in all font sizes, that's too
small for larger fonts. Make the border size depend on the font size for
larger fonts to make sure that the rendered glyphs are indeed
independent.  The introduced GLYPH_SPACE_RATIO = 8 constant is the
smallest value where I don't see overflows anymore for large fonts.

With this, not even the bugdoc renders correctly when presenting, but
also the text is OK when the font size is set to the max allowed by the
UI (96pt).

Change-Id: I52aa5c4444583fc0467291463697a63f2b8fc86c