add babylon/core/log_trace.h: tracing utilities for emscripten
add babylon/core/log_trace.h: tracing utilities for emscripten

(disabled by default, enabled on a file per file basis)

samdauwe/BabylonCpp,samdauwe/BabylonCpp,samdauwe/BabylonCpp,samdauwe/BabylonCpp