gamma correction fallback for -nosrgb

[demo_prior] / sdr / gamma.p.glsl

uniform sampler2D tex;

void main()
{
        vec3 texel = texture2D(tex, gl_TexCoord[0].st).rgb;
        vec3 col = pow(texel, vec3(1.0 / 2.2));
        gl_FragColor = vec4(col, 1.0);
}