Add built-in ShaderFlag for skinning vertices;
Add built-in ShaderFlag for skinning vertices;

Now you can pass in the { skinned = true } flag when creating a Shader
to get skinning behavior without needing to write it yourself.  The
default is still false.

Need to profile to make sure the if is getting optimized out.

bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr