Inverse cart: speed-up computation by 33%

Saves 2 sincos() and 1 atan2() calls.

With the following bench
```
 #include "proj.h"
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>

int main(int argc, char* argv[])
{
    if( argc != 2 ) {
        fprintf(stderr, "Usage: bench_inv_cart fwd/inv\n");
        exit(1);
    }
    PJ* p = proj_create(0, "+proj=cart");
    const int dir = strcmp(argv[1], "inv") == 0 ? PJ_INV : PJ_FWD;
    PJ_COORD coord;
    if( dir == PJ_FWD )
    {
        coord.xyz.x = 3.14159/2;
        coord.xyz.y = 3.14159/2;
        coord.xyz.z = 100;
    }
    else
    {
        coord.xyz.x = 3e6;
        coord.xyz.y = 3e6;
        coord.xyz.z = 3e6;
    }
    for(int i = 0; i < 10* 1024 * 1024; i++ )
    {
        proj_trans(p, dir, coord);
    }

    proj_destroy(p);
    return 0;
}
```

On Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
Time before: 2.37s
Time after:  1.57s

1 files changed, 6 insertions, 0 deletions

diff --git a/test/gie/more_builtins.gie b/test/gie/more_builtins.gie
index a3e9d398..323b85c1 100644
--- a/test/gie/more_builtins.gie
+++ b/test/gie/more_builtins.gie
@@ -840,6 +840,12 @@ expect      180 0 0
 accept      0 0 0
 expect      0 90 -6356752.314140356
 
+accept      0 0 1e-6
+expect      0 90 -6356752.314139356
+
+accept      0 0 -1e-6
+expect      0 -90 -6356752.314139356
+
 
 -------------------------------------------------------------------------------
 # Test handling of endianness of NTv2 grids