Auto merge of #1255 - MortimerGoro:webgl_get_x_bits, r=glennw
Auto merge of #1255 - MortimerGoro:webgl_get_x_bits, r=glennw

Enable gl::XXX_BITS constants in WebGL getParameter command.

Required to pass some WebGL conformance tests. See https://github.com/servo/servo/pull/16867

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