Add self-feedback system and water shader

Features:
- Self-feedback rendering system for shaders with feedback loops
- Automatic FBO/texture management for feedback
- Metadata parser detects iChannel feedback configuration
- Adaptive render loop (with/without feedback)
- Water shader from Shadertoy (adapted and working)
- Fixed variable initialization issues in shader code

Technical details:
- FBO creation/destruction on shader switch
- Texture binding to iChannel0-3 based on metadata
- Auto-resize feedback buffers on window resize
- Cleanup on exit and shader switch

Files:
- src/main.cpp: Feedback system implementation
- shaders/water.glsl: Water shader with fixes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

shaders/water.glsl

@@ -1,5 +1,5 @@
 // Name: Water
-// Author: diatribes
+// Author: diatribes (FabriceNeyret2)
 #version 330 core
 precision highp float;
 
@@ -14,18 +14,22 @@ uniform float iTime;
     thanks!!  :D
 
     If it doesn't display correctly, change line 17 "r/r" to "vec3(1)"
-
 */
 
 void mainImage( out vec4 o, vec2 u ) {
-    float s=.002,i,n;
+    float s=.002, i=0., n;  // FIXED: Initialize i=0
     vec3 r = vec3(iResolution.xy, iResolution.x/iResolution.y);
     vec3 p = vec3(0);
     u = (u-r.xy/2.)/r.y-.3;
-    for(o *= i; i++ < 32. && s > .001;o += vec4(5,2,1,0)/max(length(u-.1), 0.001))
-        for (p += vec3(u*s,s),s = 1. + p.y,
-            n =.01; n < 1.;n+=n)
+
+    o = vec4(0);  // FIXED: Initialize output to black
+
+    for(; i < 32. && s > .001; i++) {
+        o += vec4(5,2,1,0)/max(length(u-.1), 0.001);
+        for (p += vec3(u*s,s), s = 1. + p.y, n =.01; n < 1.; n+=n) {
             s += abs(dot(sin(p.z+iTime+p / n), vec3(1))) * n*.1;
+        }
+    }
     o = tanh(o/5e2);
 }