Auto merge of #13102 - anholt:webgl-is-program, r=emilio

webgl: Add isProgram() support.

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webgl: Add isProgram() support.

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There's one failure still, where a deleted program should still be
considered to be a program until it's unbound.  However, I recently
made it so that we unbind at delete time, and we may need to partially
back that change out.

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