vrend: Introduce external flag to make COPY_TRANSFER3D bidirectional

Open Oleksandr Gabrylchuk requested to merge samael/virglrenderer:vrend-add-flag-COP into main

If feature VIRGL_CAP_V2_COPY_TRANSFER_BOTH_DIRECTIONS is set virglrenderer does not import/export DMA buffers but use host memory access to update the guest buffers. This optimization is not compatible with current implementation of virtio devices for COQOS and crosvm hypervisors. Introduce external feature flag to enable CCMD COPY_TRANSFER3D bidirectional.

cc @olv

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