Surface-to-surface blits were broken (would draw to screen instead),
and the SDLSurface backend wouldn't even compile.
You know, I thought I tested each backend when I made those changes
to the API...
This fixes the Texture backend bug that made the program take forever
to shut down:
The problem was that the font system would try to load a glyph that's
0 pixels wide/tall (likely the space character), which SDL2 didn't
like, so it would fail to allocate the texture, causing
Backend_CreateSurface, and by extension Backend_CreateGlyph, to
return a NULL. Later, upon shutdown, the font system would pass this
NULL to Backend_FreeGlyph, causing NULL pointer dereferences that
make the program take forever to shut down.
Personally, I think passing NULLs to the backend is valid behaviour,
so I've added a bunch of sanity checks to make sure they're never
dereferenced.
