Use texelFetch instead of texture for indexed mode shaders

Fixes various weird issues when palettes have empty slots, and removes unnecessary calculations.

src/Shaders/Effects/Palettize.gdshader

@@ -14,7 +14,7 @@ vec4 swap_color(vec4 color) {
 
 	int n_of_colors = textureSize(palette_texture, 0).x;
 	int color_index = find_index(color, n_of_colors, palette_texture);
-	return texture(palette_texture, vec2(float(color_index) / float(n_of_colors), 0.0));
+	return texelFetch(palette_texture, ivec2(color_index, 0), 0);
 }
 
 void fragment() {

src/Shaders/FindPaletteColorIndex.gdshaderinc

@@ -2,8 +2,7 @@ int find_index(vec4 color, int n_of_colors, sampler2D palette_texture) {
 	int color_index = 0;
 	float smaller_distance = distance(color, texture(palette_texture, vec2(0.0)));
 	for (int i = 0; i <= n_of_colors; i++) {
-		vec2 uv = vec2(float(i) / float(n_of_colors), 0.0);
-		vec4 palette_color = texture(palette_texture, uv);
+		vec4 palette_color = texelFetch(palette_texture, ivec2(i, 0), 0);
 		float dist = distance(color, palette_color);
 		if (dist < smaller_distance) {
 			smaller_distance = dist;

src/Shaders/IndexedToRGB.gdshader

@@ -7,16 +7,16 @@ uniform sampler2D palette_texture : filter_nearest;
 uniform sampler2D indices_texture : filter_nearest;
 
 void fragment() {
-	float index = texture(indices_texture, UV).r;
+	float index = texture(indices_texture, UV).r * 255.0;
 	if (index <= EPSILON) { // If index is zero, make it transparent
 		COLOR = vec4(0.0);
 	}
 	else {
 		float n_of_colors = float(textureSize(palette_texture, 0).x);
-		index -= 1.0 / 255.0;
-		float index_normalized = ((index * 255.0)) / n_of_colors;
-		if (index_normalized + EPSILON < 1.0) {
-			COLOR = texture(palette_texture, vec2(index_normalized + EPSILON, 0.0));
+		index -= 1.0;
+		float index_normalized = index / n_of_colors;
+		if (index < n_of_colors) {
+			COLOR = texelFetch(palette_texture, ivec2(int(index), 0), 0);
 		}
 		else {
 			// If index is bigger than the size of the palette, make it transparent.

src/Shaders/SetIndices.gdshader

@@ -8,7 +8,7 @@ uniform sampler2D palette_texture : filter_nearest;
 
 void fragment() {
 	vec4 color = texture(rgb_texture, UV);
-	if (color.a <= 0.01) {
+	if (color.a <= 0.0001) {
 		COLOR.r = 0.0;
 	}
 	else {