Fallback for if GL_ARB_clip_control not available

If GL_ARB_clip_control not available then [znear, zfar] -> [1, -1] instead of [1, 0] in normalized device coordinates

Thus, Z is still reversed, but not shifted, clipping should still work properly, and conversion to window coordinates does not cause accuracy regression compared to non-reversed Z rendering.

src/render/render_gl2.c

@@ -119,6 +119,9 @@ void mjr_readPixels(unsigned char* rgb, float* depth,
         int N_pixels = viewport.width * viewport.height;
         for (int i = 0; i < N_pixels; i++) depth[i] = 1.0 - depth[i]; // Reverse the reversed Z buffer
       }
+      else if (!mjGLAD_GL_ARB_clip_control) {
+        mju_warning("ARB_clip_control unavailable while mjDEPTH_ZEROFAR requested, depth accuracy will be limited");
+      }
     }
   }
 
@@ -163,11 +166,13 @@ void mjr_readPixels(unsigned char* rgb, float* depth,
     if (depth) {
       glReadPixels(viewport.left, viewport.bottom, viewport.width, viewport.height,
                    GL_DEPTH_COMPONENT, GL_FLOAT, depth);
-
       if (con->readDepthMap == mjDEPTH_ZERONEAR) {
         int N_pixels = viewport.width * viewport.height;
         for (int i = 0; i < N_pixels; i++) depth[i] = 1.0 - depth[i]; // Reverse the reversed Z buffer
       }
+      else if (!mjGLAD_GL_ARB_clip_control) {
+        mju_warning("ARB_clip_control unavailable while mjDEPTH_ZEROFAR requested, depth accuracy will be limited");
+      }
     }
 
     // restore currentBuffer