gltfpack: Implement support for JSON report output
gltfpack: Implement support for JSON report output

A new option, -r file, can be used to output a JSON report for the
processing of the scene.

The report contains summary scene information (counts of various
objects), render complexity (number of draw calls and triangles) and
data sizes (JSON/binary sizes plus a breakdown of buffer data).

Unlike -v/-vv which is purely informational and can change at any point
in time, the schema for -r should be reasonably stable from version to
version.

Fixes #162.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer