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Preliminary texture mapping support.

Maps a **texture** onto triangles being built
from **vertices** according to the chosen **formation**.

Optional **indices** can be used to avoid rearrangement of vertex lists,
otherwise the vertex list is processed consecutively, i.e. as if **indices**
are ascending numbers starting at zero.

Either the number of **indices** (if non NULL) or the number of **vertices** is
specified by **num** and has to be three at least. If the chosen **formation**
is DTTF_LIST it also has to be a multiple of three.