Clarify draw calls without fragment outputs for WebGL 1.0

WebGL 1.0, section "Fragment shader output":
> If a fragment shader writes to neither `gl_FragColor` nor `gl_FragData`, the values of the fragment colors following shader execution are untouched.

WebGL 2.0, section "Draw Buffers":
> If any draw buffer with an attachment does not have a defined fragment shader output, draws generate `INVALID_OPERATION`, unless all 4 channels of `colorMask` are set to false.

In practice, the WebGL 2.0 rule also applies to WebGL 1.0 contexts.