Added LZ4 and benchmark program

[unsettledgames]

• Added LZ4 support in cortolib
• Built a simple benchmark program (cortobenchmark) to test the performances of different entropy algorithms. Added an optional build target to CMakeLists.txt
• Added LZ4 support to the emscripten library
• Updated and fixed emscripten build scripts to support new sdl version, extended them to support LZ4
• Added simple material loading from .crt files in the THREE.js loader