IE 11 workarounds for render states

Test this with IE 11.  It should fix:

* INVALID_OPERATION: disable: Enum SAMPLE_COVERAGE is not currently supported
* INVALID_OPERATION: enable: Enum DITHER is not currently supported

This removes the `dither` render state, which I believe is a no-op on all implementations and is not going to be included in glTF.  I am very surprised it is in ES 2.0 and WebGL.