patched SDL_rwops_zzip so it supports size() on the stream

TTF_OpenFontRW was giving segfaults; turns out the offending line was:
stream->size = (unsigned long)(SDL_RWsize(src) - position);
with SDL_rwops.h:#define SDL_RWsize(ctx) (ctx)->size(ctx)
and SDL_RWopsFromZZIP didn't support size() on the stream
(now it does)