With a GPU accelerator every frame of a batch rendered in parallel, each
holding its own working textures, so a 30-frame 1080p batch could take
the whole card (20+ GB on an RTX 3090) and run shared-memory GPUs out of
memory. Cx::frames now renders GPU frames in consecutive chunks of 3
(EFFECTCRAFT_GPU_INFLIGHT overrides it), the same cap as the export
scheduler in #898; output order and the CPU path are unchanged.
Fixes#879
Matej Kozlovský
742472440e