Add hsv.gdshaderinc to shaderlibrary, provide an example and update README.md - implement #12

shaderlib/hsv.gdshaderinc

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+
+// rgb2hsv and hsv2rgb functions adapted from https://godotshaders.com/shader/hsv-adjustment/
+
+// Convert RGB to HSV (hue, saturation, brightness)
+vec4 rgb2hsv(vec4 c) {
+	vec4 K = vec4(0.0, -1.0 / 3.0, 2.0 / 3.0, -1.0);
+	vec4 p = mix(vec4(c.bg, K.wz), vec4(c.gb, K.xy), step(c.b, c.g));
+	vec4 q = mix(vec4(p.xyw, c.r), vec4(c.r, p.yzx), step(p.x, c.r));
+	float d = q.x - min(q.w, q.y);
+	float e = 1.0e-10;
+	return vec4(abs(q.z + (q.w - q.y) / (6.0 * d + e)), d / (q.x + e), q.x, c.a);
+}
+
+// Convert HSV back to RGB (red, green, blue)
+vec4 hsv2rgb(vec4 c) {
+	vec4 K = vec4(1.0, 2.0 / 3.0, 1.0 / 3.0, 3.0);
+	vec3 p = abs(fract(c.xxx + K.xyz) * 6.0 - K.www);
+	vec3 rgb =  c.z * mix(K.xxx, clamp(p - K.xxx, 0.0, 1.0), c.y);
+	return vec4(rgb.r, rgb.g, rgb.b, c.a);
+}
+
+// Offset the hue, saturation and brightness of a RGB color
+vec4 hsv_offset(
+	vec4 rgba,
+	float offset_hue,
+	float offset_saturation,
+	float offset_brightness
+) {
+	vec4 c = rgb2hsv(rgba);
+	c.x += offset_hue;
+	c.y += offset_saturation;
+	c.z += offset_brightness;
+	return hsv2rgb(c);
+}
+
+// Multiply the hue, saturation and brightness of a RGB color
+vec4 hsv_multiply(
+	vec4 rgba,
+	float mult_hue,
+	float mult_saturation,
+	float mult_brightness
+) {
+	vec4 c = rgb2hsv(rgba);
+	c.x *= mult_hue;
+	c.y *= mult_saturation;
+	c.z *= mult_brightness;
+	return hsv2rgb(c);
+}