Clarify draw calls without fragment outputs for WebGL 1.0

[lexaknyazev]

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.