Fix some image effects resulting in wrong colors when the image has non-opaque pixels

src/Shaders/Effects/OffsetPixels.gdshader

@@ -1,4 +1,5 @@
 shader_type canvas_item;
+render_mode unshaded;
 
 uniform sampler2D selection : filter_nearest;
 uniform vec2 offset = vec2(0.0); // In pixels

src/Shaders/Effects/Palettize.gdshader

@@ -1,6 +1,7 @@
 // Maps the color of the input to the nearest color in the selected palette.
 // Similar to Krita's Palettize filter
 shader_type canvas_item;
+render_mode unshaded;
 
 uniform sampler2D palette_texture : filter_nearest;
 uniform sampler2D selection : filter_nearest;

src/Shaders/Effects/Pixelize.gdshader

@@ -5,6 +5,7 @@ https://godotshaders.com/shader/pixelate-2/
 This shader is under MIT license
 */
 shader_type canvas_item;
+render_mode unshaded;
 
 uniform uvec2 pixel_size = uvec2(4);
 uniform sampler2D selection : filter_nearest;

src/Shaders/Effects/Posterize.gdshader

@@ -1,5 +1,6 @@
 // https://godotshaders.com/shader/color-reduction-and-dither/
 shader_type canvas_item;
+render_mode unshaded;
 
 uniform sampler2D selection : filter_nearest;
 uniform float colors : hint_range(1.0, 255.0) = 2.0;

src/Shaders/Effects/Rotation/OmniScale.gdshader

@@ -1,6 +1,7 @@
 // No AA version from https://github.com/deakcor/godot-omniscale/blob/5dfee6e89cd955dd01dccfe70c9979f9b55bb1bf/OmniScale.shader
 // Edited slightly by Overloaded to add rotation support for Pixelorama
 shader_type canvas_item;
+render_mode unshaded;
 
 //#version 130
 

src/Shaders/Effects/Rotation/cleanEdge.gdshader

@@ -25,6 +25,7 @@ OTHER DEALINGS IN THE SOFTWARE.
 
 
 shader_type canvas_item;
+render_mode unshaded;
 #include "res://src/Shaders/Effects/Rotation/CommonRotation.gdshaderinc"
 //enables 2:1 slopes. otherwise only uses 45 degree slopes
 #define SLOPE