hasBufferView method

Whether accessorsaccessorIndex names a bufferView.

False is legal on its own — every element then reads as zero, which is exactly what a sparse accessor's own base is meant to be. It is also the shape a KHR_draco_mesh_compression/EXT_meshopt_compression primitive's fallback accessor takes when an exporter left the real data in the extension's own buffer and named no fallback at all — fmt-15's own reason a caller needs to ask this before reading, rather than reading and getting a silent, degenerate zero mesh back.