Added shaders

Shaders/GodotRetro/Screen Shaders/SimpleGrain.gdshader

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+//SHADER ORIGINALY CREADED BY "juniorxsound" FROM SHADERTOY
+//MODIFIED AND PORTED TO GODOT BY AHOPNESS (@ahopness)
+//LICENSE : CC0
+//COMATIBLE WITH : GLES2, GLES3, WEBGL
+//SHADERTOY LINK : https://www.shadertoy.com/view/ldScWw
+
+shader_type canvas_item;
+
+uniform sampler2D SCREEN_TEXTURE : hint_screen_texture, filter_linear_mipmap;
+
+uniform float amount :hint_range(0.0, 0.4) = 0.1;
+
+float grain (vec2 st, float time){
+	return fract(sin(dot(st.xy, vec2(17.0,180.)))* 2500. + time);
+}
+
+void fragment(){
+	//Coords
+	vec2 uv =  FRAGCOORD.xy / (1.0 / SCREEN_PIXEL_SIZE).xy;
+
+	//Produce some noise based on the coords
+	vec3 grainPlate = vec3(grain(uv, TIME));
+
+	//Get the image
+	vec4 img = texture(SCREEN_TEXTURE, uv);
+
+	//Mix the two signals together
+	vec3 mixer = mix(img.rgb, grainPlate, amount);
+
+
+	COLOR = vec4(mixer,1.0);
+}