regl has a bug with sRGB texture

[xiaoiver]

PR: regl-project/regl#523

The bottom baseColor is correct while the top is wrong:

So we must do this conversion manually for now until regl fix this bug.

// pbr-frag.glsl
vec4 SRGBtoLINEAR(vec4 srgbIn)
{
    vec3 bLess = step(vec3(0.04045),srgbIn.xyz);
    vec3 linOut = mix( srgbIn.xyz/vec3(12.92), pow((srgbIn.xyz+vec3(0.055))/vec3(1.055),vec3(2.4)), bLess );
    return vec4(linOut,srgbIn.w);;
}