Fix gcc/clang warnings for -Wall/-Wextra
Fix gcc/clang warnings for -Wall/-Wextra

Fix -Wunused-parameter warnings by explicitly marking unused arguments
as such, and -Wmissing-field-initializers by specifying all 3 struct
fields for jsmn_parser.

jkuhlmann/cgltf,jkuhlmann/cgltf,jkuhlmann/cgltf