Let grdimage share -T with grdview

[PaulWessel]

This PR is creating a function of out grdview -T so it can be called from grdimage as well as grdview. We can consider extending this to handing images (which grdimage may be fed) but for now it is like grdview -T.

Tested on @anbj script and seems to work fine - no test issues. However, need @joa-quim to try this from Julia and passing memory since it is at least possible that something different needs to be done when I call goto basemap_and_free he end.

[joa-quim]

No problems from the Julia side.

[anbj]

Fantastic - thank you Paul!

Just a PS: Skip nodes with NaN/NaN values transparent: -Q has no effect if -T is used, and basically replaced with T+s. Worth noting in the docs?

[anbj]

One more note: -T+oblack+s -t50 makes the cells transparent, as expected. -T+oblack@50+s -t50 makes nothing transparent ('cancels' the -t50). I don't know if -T+oblack should even accept a @50. Just testing!

[PaulWessel]

You could see if #7322 makes things more stable?

[anbj]

(Getting messy. Tell me if you want a separate issue for this.)

-T+oblack+s -t40 - works as expected.

These do not;

-T+oblack@20+s -t50:

-T+oblack@20+s: