De-allocate caches for large glyphs when trimming memory

Currently, font renderers eliminate some texture caches when
memory is trimmed. This change makes it go further by eliminating the
large-glyph caches for all font renderers. These caches are
only allocated as needed, but continue to consume large amounts of
memory (CPU and GPU) after that allocation. De-allocating this memory
on a trim operation should prevent background apps from holding onto
this memory in the possible case that they have allocated it by drawing
large glyphs.

Change-Id: Id7a3ab49b244e036b442d87252fb40aeca8fdb26