Previously, the font batching system was very shoddy: basically,
glyph bitmaps would be cached to system memory, and uploaded
on-demand to the GPU.
The OpenGL3, OpenGLES2, and SDLTexture backends relied on
`cute_spritebatch.h` to handle batching. While nice, this library is
overkill: it's intended for managing a whole game, not just a couple
of glyphs. It also depends on C++11, which is something I'd rather be
without.
Being so complex, it appears that internally it spams the backend
when merging atlases together. According to bug reports, this causes
the game to skip a lot of frames.
Instead, I've ditched the system-memory-side caching, and made the
game maintain its own atlas. Unlike `cute_spritebatch.h`, this one
doesn't purge glyphs after they've gone unused for 30(?) seconds -
instead, glyphs are only purged when room in the atlas runs out, at
which point the oldest glyph is replaced. This method doesn't involve
creating or destroying atlases at runtime, avoiding the overhead of
whatever OpenGL/DirectX do internally when that happens.
Currently only the OpenGL3, OpenGLES2, and Software renderers have
been updated with this - the others will fail to build.
I know immediate-mode renderers won't benefit from this, but it
replaces the leaky old caching system, so it's still an improvement.
The Wii U has a very limited pool of memory for render targets
(32MB), so we should only use it if we have to.
This 'fixes' a bug in the enhanced branch, where if you use 2x
sprites at 854x480, the third line of the text box will be corrupted
(text will appear on the second line instead, and be black instead of
white).
The other renderers haven't been updated for the API change yet.
