Improve shadow performance: halve resolution, 9x fewer PCF taps, throttle depth pass

- SHADOW_MAP_SIZE 2048→1024: 4x fewer pixels rasterized in depth pass
- Replace 9-tap manual PCF loop with single hardware PCF tap in all 4 receiver
  shaders (terrain.frag, wmo_renderer, m2_renderer, character_renderer).
  GL_LINEAR + GL_COMPARE_REF_TO_TEXTURE already gives 2×2 bilinear PCF per
  tap for free, so quality is maintained while doing 9x fewer texture fetches.
- Throttle shadow depth pass to every 2 frames; OpenGL depth texture persists
  between frames so receivers always have a valid shadow map. 1-frame lag at
  60 fps is invisible.

include/rendering/renderer.hpp

@@ -223,7 +223,7 @@ private:
     void shutdownPostProcess();
 
     // Shadow mapping
-    static constexpr int SHADOW_MAP_SIZE = 2048;
+    static constexpr int SHADOW_MAP_SIZE = 1024;
     uint32_t shadowFBO = 0;
     uint32_t shadowDepthTex = 0;
     uint32_t shadowShaderProgram = 0;
@@ -231,6 +231,7 @@ private:
     glm::vec3 shadowCenter = glm::vec3(0.0f);
     bool shadowCenterInitialized = false;
     bool shadowsEnabled = false;
+    int shadowFrameCounter_ = 0;  // throttle: only re-render depth map every 2 frames
 
 public:
     void setShadowsEnabled(bool enabled) { shadowsEnabled = enabled; }