Change Futatabi frames to be cached as textures instead of in system memory.

The JPEGs are now decoded into PBO bounce buffers, which saves a lot of CPU
time (copying is asynchronous, and done by the GPU -- plus we save a copy
into a staging buffer).

Similarly, keeping the cache in textures allows the driver (if it wants!)
to keep it in VRAM, saving repeated uploading if the same frame is used
multiple times.

CPU usage is down from 1.05 to 0.60 cores on my machine, when not playing.
More importantly, the 99-percentile player queue status is extremely much
better.