Make writing to gl_FragData[n != 0] fail compilation in WebGL 2

This should clear any ambiguity caused by some parts of the GLES 3.0
specification suggesting that writing to multiple outputs in GLSL ES 1.0
shaders would be possible, while section 4.2.1 says that calling
DrawBuffers only sets the output buffer for one output color for
GLSL ES 1.0 shaders.