Added shaders

Shaders/GodotRetro/Screen Shaders/Aditional Shaders/Dithering.gdshader

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+//SHADER ORIGINALY CREADED BY "abelcamarena" FROM SHADERTOY
+//MODIFIED AND PORTED TO GODOT BY AHOPNESS (@ahopness)
+//LICENSE : CC0
+//COMATIBLE WITH : GLES2, GLES3, WEBGL
+//SHADERTOY LINK : https://www.shadertoy.com/view/tsKGDm
+
+shader_type canvas_item;
+
+uniform sampler2D SCREEN_TEXTURE : hint_screen_texture, filter_linear_mipmap;
+
+uniform float SCREEN_WIDTH = 320.; // Lower num - bigger pixels (this will be the screen width)
+uniform float COLOR_FACTOR :hint_range(0., 10.) = 4.;   // Higher num - higher colors quality
+uniform float DITHERING_STRENTH :hint_range(0., .07) = 0.005; // Be carefull with this one, dithering can get messy really easily
+
+int PSXDither(ivec2 fragcoord) {
+    const int dither_table[16] = {
+        -4, +0, -3, +1, 
+        +2, -2, +3, -1, 
+        -3, +1, -4, +0, 
+        +3, -1, +2, -2
+    };
+    
+    int x = fragcoord.x % 4;
+    int y = fragcoord.y % 4;
+    
+    return dither_table[y * 4 + x];
+}
+
+void fragment(){                  
+    // Reduce pixels            
+    vec2 size = SCREEN_WIDTH * SCREEN_PIXEL_SIZE.xy/SCREEN_PIXEL_SIZE.x;
+    vec2 coor = floor( UV * size) ;
+    vec2 uv =  FRAGCOORD.xy / (1.0 / SCREEN_PIXEL_SIZE).xy;
+    
+    // Get source color
+    vec3 col = texture(SCREEN_TEXTURE, uv).xyz;     
+    
+    // Dithering
+    col += float(PSXDither(ivec2(FRAGCOORD.xy))) * DITHERING_STRENTH;
+    
+    // Reduce colors    
+    col = floor(col * COLOR_FACTOR) / COLOR_FACTOR;    
+    
+    // Output to screen
+    COLOR = vec4(col,1.);
+}