gpu_ctx: add NGLI_FEATURE_BUFFER_MAP
gpu_ctx: add NGLI_FEATURE_BUFFER_MAP

This will allow to hint if the underlying backend support mapping
buffers (including persistent mappings).

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl