Fix misleading warnings for OES_standard_derivatives

[jjhembd]

When a Custom Shader fails to compile, the error message begins with an irrelevant warning:

The OES_standard_derivatives extension is only available in WebGL1 contexts. In a WebGL2 context, the derivatives functionality is available by default. However, several of our fragment shaders (written in ES300) still enabled this extension conditionally:

#ifdef GL_OES_standard_derivatives
    #extension GL_OES_standard_derivatives : enable
#endif

The middle line was then moved to the top of the file in ShaderSource without the #ifdef block, resulting in a warning on WebGL2 contexts.

This PR simplifies our handling of the OES_standard_derivatives extension. Since it is very commonly used, but only needed in WebGL1, I removed all the code to enable the extension in individual shaders. I then always enable the extension (if available) in demodernizeShader.

The above error message now becomes:

which better emphasizes the relevant error.

The errors can be reproduced with this local Sandcastle

[cesium-concierge]

Thanks for the pull request @jjhembd!

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[lilleyse]

@jjhembd thanks, the approach here makes sense. I confirmed that the warning no longer shows up and that sandcastle examples using standard derivatives still work in WebGL 1 and 2.