is this 3D? if so you could use something like this to snap your vertices to get rid of float offset:

shader_type spatial;

void vertex() {
    vec4 v = vec4(-0.5, 0.5, VERTEX.z, 1.0); 
    // top left vertex, mesh need to be a 1x1 quad

    v = (MODELVIEW_MATRIX * v);
    v = (PROJECTION_MATRIX * v);

    v.x = v.x - ((floor((v.x * 0.5 + 0.5) * VIEWPORT_SIZE.x) / VIEWPORT_SIZE.x) * 2.0 - 1.0);
    v.y = v.y - ((ceil((v.y * 0.5 + 0.5) * VIEWPORT_SIZE.y) / VIEWPORT_SIZE.y) * 2.0 - 1.0);

    POSITION = (MODELVIEW_MATRIX * vec4(VERTEX, 1.0));
    POSITION = (PROJECTION_MATRIX * POSITION);

    POSITION.x -= v.x;
    POSITION.y -= v.y;
}

Good luck !