Is your feature request related to a problem? Please describe.

Currently developer can only specify multiple render target using gl_FragData with glsl100 syntax, but gl_FragData is not supported in WebGL2.

In Unity ShaderLab, you specify mrt by sematic keywords SV_Target, SV_Target0, SV_Target1, ...

fixed4 frag (v2f i) : SV_Target
{
    return fixed4(i.uv, 0, 0);
}

// or wrapped in struct
struct fragOutput {
    fixed4 color : SV_Target;
    fixed4 color1 : SV_Target1;
};
fragOutput frag (v2f i)
{
    fragOutput o;
    o.color = fixed4(i.uv, 0, 0);
    o.color1 = fixed4(i.uv, 1, 0);
    return o;
}

Describe the solution you'd like

A complete MRT support in WebGL1 and Webgl2

Syntax

...
// webgl1
void main(v2f input) {
   // mrt
  gl_FragData[0] = vec4(1.,2.,3.,4.);
  gl_FragData[1] = vec4(1.,2.,3.,4.);

  // normal
  gl_FragColor = vec4(1.0);
}

// webgl2
// normal
vec4 main(v2f input) {
  vec4 color = vec4(1.0);
  return color;
}

// mrt
struct mrt {
  layout(location = 0) vec4 fragColor0;
  layout(location = 1) vec4 fragColor1;
}

mrt main(v2f input) {
  mrt output;
  output.fragColor0= xxx;
  output.fragColor1 = xxxx;
  return output;
}
...

All syntax above are supported.

the properties of output struct in fragment shader must all be vec4 property qualified by layout and location.