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, 4 insertions, 0 deletions

diff --git a/src/grids.hpp b/src/grids.hpp
index d060fc95..459bde07 100644
--- a/src/grids.hpp
+++ b/src/grids.hpp
@@ -45,6 +45,10 @@ struct ExtentAndRes {
     double north;      // in radian for geographic, in CRS units otherwise
     double resX;       // in radian for geographic, in CRS units otherwise
     double resY;       // in radian for geographic, in CRS units otherwise
+    double invResX;    // = 1 / resX;
+    double invResY;    // = 1 / resY;
+
+    void computeInvRes();
 
     bool fullWorldLongitude() const;
     bool contains(const ExtentAndRes &other) const;