Remove duplicate definitions of linear2srgb functions in shaders

kitty/linear2srgb.glsl

@@ -0,0 +1,15 @@
+float srgb2linear(float x) {
+    // sRGB to linear conversion
+    float lower = x / 12.92;
+    float upper = pow((x + 0.055f) / 1.055f, 2.4f);
+
+    return mix(lower, upper, step(0.04045f, x));
+}
+
+float linear2srgb(float x) {
+    // Linear to sRGB conversion.
+    float lower = 12.92 * x;
+    float upper = 1.055 * pow(x, 1.0f / 2.4f) - 0.055f;
+
+    return mix(lower, upper, step(0.0031308f, x));
+}