Fix shader error on OSX.

[psigen]

npm test is currently failing with an 'exceeded max number of retries' and a segfault that OSX reports as node failing to exit cleanly.

$ npm test

> bgfx@0.89.0 test /Users/pkv/Desktop/node-xr/node-bgfx
> jest --bail

 PASS  test/utils/util.test.js
 PASS  test/index.test.js

 RUNS  test/vertex/VertexBuffer.test.js
-[_MTLCommandEncoder dealloc]:75: failed assertion `Command encoder released without endEncoding'
Abort trap: 6