Activate WMO/char/M2 render loop, purge dead GL block, add underwater overlay

- renderWorld() now calls wmoRenderer, characterRenderer, m2Renderer (+smoke,
  particles) in the correct opaque→transparent order; water moved after all
  opaques; per-subsystem timing active in live path
- Deleted the 310-line #if 0 GL stub block and removed #include <GL/glew.h>
  (last GL reference in renderer.cpp)
- Full-screen overlay pipeline (postprocess.vert + overlay.frag, alpha blend,
  no depth test/write) for underwater tint; lazily initialized, renders a blue
  tint when camera is meaningfully below the water surface; canal vs open-water
  tint colours preserved from original design
- overlay.frag.glsl / overlay.frag.spv added

assets/shaders/overlay.frag.glsl

@@ -0,0 +1,14 @@
+#version 450
+
+// Full-screen color overlay (e.g. underwater tint).
+// Uses postprocess.vert.glsl as vertex shader (fullscreen triangle, no vertex input).
+
+layout(push_constant) uniform Push {
+    vec4 color; // rgb = tint color, a = opacity
+} push;
+
+layout(location = 0) out vec4 outColor;
+
+void main() {
+    outColor = push.color;
+}