GeoTIFF grid reading: perf improvements (fixes #2785)

With this commit, and the 2 previous ones, given mytest.cpp
```

int main()
{
    PJ* pj = proj_create(nullptr, "+proj=vgridshift +grids=us_nga_egm96_15.tif");
    for( int i = 0; i < 5*1000*1000; i++)
    {
        PJ_COORD coord;
        coord.lpz.lam = 0;
        coord.lpz.phi = 0;
        coord.lpz.z = 0;
        proj_trans(pj, PJ_FWD, coord);
    }
    return 0;
}
```

we get a x2 speedup

Before:
```
$ PROJ_LIB=data:$HOME/proj/PROJ-data/us_nga LD_LIBRARY_PATH=src/.libs  hyperfine --warmup 1 'taskset -c 11 ./mytest'
Benchmark #1: taskset -c 11 ./mytest
  Time (mean ± σ):      1.950 s ±  0.014 s    [User: 1.945 s, System: 0.005 s]
  Range (min … max):    1.937 s …  1.971 s
```

After:
```
$ PROJ_LIB=data:$HOME/proj/PROJ-data/us_nga LD_LIBRARY_PATH=src/.libs  hyperfine --warmup 1 'taskset -c 11 ./mytest'
Benchmark #1: taskset -c 11 ./mytest
  Time (mean ± σ):     984.4 ms ±   3.1 ms    [User: 977.0 ms, System: 7.2 ms]
  Range (min … max):   979.3 ms … 990.5 ms
```

1 files changed, 15 insertions, 0 deletions

diff --git a/include/proj/internal/lru_cache.hpp b/include/proj/internal/lru_cache.hpp
index b7aff6b9..b2e997b3 100644
--- a/include/proj/internal/lru_cache.hpp
+++ b/include/proj/internal/lru_cache.hpp
@@ -160,6 +160,21 @@ class Cache {
     keys_.splice(keys_.begin(), keys_, iter->second);
     return iter->second->value;
   }
+
+  /**
+   *	The const reference returned here is only
+   *    guaranteed to be valid till the next insert/delete
+   */
+  const Value* getPtr(const Key& k) {
+    Guard g(lock_);
+    const auto iter = cache_.find(k);
+    if (iter == cache_.end()) {
+      return nullptr;
+    }
+    keys_.splice(keys_.begin(), keys_, iter->second);
+    return &(iter->second->value);
+  }
+
   /**
    * returns a copy of the stored object (if found)
    */