Blender Git Loki

Jpeg: Fix write past array boundary

Was happening when image buffer had cryptomatte pass, which can easily
exceed 530 bytes used by the buffer.

Now default buffer is bumped to 1K, and also allowed to be heap-allocated
when really need bigger buffer.

Possible optimization is to allocate buffer once, but in practice those
re-allocations will not happen often, so keeping code simpler is not an
issue. Just something for a rainy day.