Expose target_error to gltfpack

[FreakTheMighty]

It would be helpful to have target_error exposed to gltfpack. If I understand this correctly, this could provide another tool for aggressively simplifying, while setting a lower floor for quality.

meshoptimizer/src/simplifier.cpp

Line 1423 in 7d8d3f7

size_t meshopt_simplifySloppy(unsigned int* destination, const unsigned int* indices, size_t index_count, const float* vertex_positions_data, size_t vertex_count, size_t vertex_positions_stride, size_t target_index_count, float target_error, float* out_result_error)

[zeux]

Right now gltfpack sets the error automatically to 0.1f (meaning up to 10% deviation) when using aggressive simplifier. The slight difficulty in exposing a setting for this is that the error thresholds are actually different between normal mode and aggressive mode, so -sa ends up doing two attempts at simplification - once with meshopt_simplify with target error 1%, and once more if necessary with meshopt_simpifySloppy with target error 10%.

Are you looking to increase or reduce the error threshold?

[FreakTheMighty]

@zeux I think the main use case would be to remove as many triangles away while preserving a specific quality. To produce LODs I might do something like set the triangle target to 0, but vary the maximum error.