Add an initial rhi implementation that mimics the opengl implementation

common/rhi/shaders/flow.frag

@@ -0,0 +1,44 @@
+#version 450
+
+// Inputs from vertex shader
+layout(location = 0) in vec2 vTexCoord;
+layout(location = 1) in vec4 vColor;
+layout(location = 2) in float vIsReflection;
+
+// Output
+layout(location = 0) out vec4 fragColor;
+
+// Uniform buffer
+layout(std140, binding = 0) uniform UniformBuffer
+{
+    mat4 viewProjectionMatrix;
+    vec3 backgroundColor;
+    float _pad0;
+    vec3 shadingColor;
+    float _pad1;
+    float reflectionUp;
+    float reflectionDown;
+    int isReflection;
+    float _pad2;
+};
+
+// Texture and sampler
+layout(binding = 1) uniform sampler2D coverTexture;
+
+void main()
+{
+    vec4 texColor = texture(coverTexture, vTexCoord);
+
+    // Apply shading: multiply texture by vColor.r to darken
+    float shadingAmount = vColor.r;
+
+    // For reflections, apply gradient fade (darker at bottom, fading to black)
+    if (vIsReflection > 0.5) {
+        // vTexCoord.y goes from 1 (top of reflection, near cover) to 0 (bottom, far from cover)
+        // We want it brightest near the cover and fading away
+        float gradientFade = mix(0.0, 0.33, vTexCoord.y);
+        shadingAmount *= gradientFade;
+    }
+
+    fragColor = vec4(texColor.rgb * shadingAmount, texColor.a);
+}