Godot 3.1: https://godotengine.org/qa/24801/chromaticaberration-shader-no-texscreen-function

Chromatic_Aberration

shader_type canvas_item;

uniform float size_x=0.005;
uniform float size_y=0.005;

uniform float r_offset_x = -0.2;
uniform float r_offset_y = 0;
uniform float g_offset_x = 0;
uniform float g_offset_y = 0;
uniform float b_offset_x = 0.2;
uniform float b_offset_y = 0;

void fragment() {
vec2 uv = SCREEN_UV;
uv-=mod(uv,vec2(size_x,size_y));

vec2 PixelSize = TEXTURE_PIXEL_SIZE;
vec4 r_val = texture(SCREEN_TEXTURE, uv + vec2(r_offset_x, r_offset_y) * PixelSize);
vec4 g_val = texture(SCREEN_TEXTURE, uv + vec2(g_offset_x, g_offset_y) * PixelSize);
vec4 b_val = texture(SCREEN_TEXTURE, uv + vec2(b_offset_x, b_offset_y) * PixelSize);

vec3 Chromatic_Aberration = vec3(r_val.r, g_val.g, b_val.b);

COLOR.rgb = Chromatic_Aberration;
//COLOR.rgb= textureLod(SCREEN_TEXTURE,uv,0.0).rgb;
}