Simple crossfade shader gl2 GLSL version 120

crossFade.frag

#version 120

uniform sampler2DRect tex0;
uniform sampler2DRect tex1;
uniform vec2 aTexSize;
uniform vec2 bTexSize;
uniform float fadeToB;

varying vec2 texCoordVarying;

void main()
{
    vec4 a = texture2DRect(tex0, texCoordVarying * aTexSize);
    vec4 b = texture2DRect(tex1, texCoordVarying * bTexSize);
  
    vec4 color = vec4(1.0, 1.0, 1.0, 1.0);
    color = mix(a, b, fadeToB);

    gl_FragColor = color;
}

crossFade.vert

#version 120

varying vec2 texCoordVarying;

void main()
{
  texCoordVarying = gl_MultiTexCoord0.xy;
	gl_Position = ftransform();
}