- The cap applied whenever an accelerator was attached, so Software Only
projects (which render on the CPU) rendered one frame at a time. It
now needs the project's GPU acceleration too, as #898 gates it.
- Only the render is capped: each window of max(cap, threads) frames
renders in chunks of the cap, then encodes in parallel, holding at
most one window of images.
- An invalid EFFECTCRAFT_GPU_INFLIGHT (0, negative, not a number, too
large) logs a warning before using the default.
- The default stays one frame: the GPU backend reports no free or total
device memory portably, so it is not sized from VRAM.
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