Feature requst: rg textures

[tigrazone]

Is it possible to use 2-channel textures, not 4 like now - for pngs not cached to ktx?
One more idea is make switcheble off ktx cache.
Normal mapping also can be 2-channel textures. Because of normal mapping usually normalized it can be used 2 channel textures and calc z like

 vec2 vNt = texture(TextureSamplers[nonuniformEXT(material.NormalTextureId)], texCoord).xy;
 vNt += vNt - vec2(1.f);
 normal = normalize( mat3(tangent, bitangent, normal) * vec3( vNt * material.NormalTextureScale, sqrt(1.f - dot(vNt, vNt)) ) );