convexe polygon around a shape

[Ofnuts]

This is called a "convex hull". There are several algorithms for this, just google. A Bezier curve is strictly enclosed in the 4 points that define it, so the thing you search is the convex hull of all the anchors and tangents in the text path (pdb.gimp_vectors_new_from_text_layer(image, layer)).

If you want an example script, my ofn-enclosing-circle does something similar: find the smallest circle that contains all the points of  a path.