Use one font for subtitles

Subtitle text have two fonts. (m_font, m_borderFont).
Each font have a texture for font atlas.

When character caches are full, they will reallocate their textures.
Video memory is fragmented by two growing textures.

Defragment is not fast enough to avoid stuttering on some system.

In this case, I think that the best way is merged into one atlas.
Many codes should be rewritten for that.

Easy way is that subtitles use one font.
Boder is drawn by drawing 8 directions with m_font. 
Of course, border is a ugly.