Remove CacheTextureLine structure from FontRenderer

CacheTextureLine was useful before we were packing the glyph
textures; it allowed simple packing of any particular texture according to
how many lines there were in a texture, and how tall those lines were.
Now that we are packing more efficiently (both horizontally and vertically
in any given texture line), it is more efficient to have
open space in every texture, removing the need for CacheTextureLine (which
now gets in the way since it limits how much can be stored in each line).

This change removes CacheTextureLine and just uses CacheTexture directly,
allowing caching of glyphs anywhere in the open space of each texture. As before,
the packing of these glyphs is determined by the CacheBlock structure, which
is a linked list of open spaces in each CacheTexture.

Change-Id: Id6f628170df0f676f8743ac7de76f2377fc6a012