added trixAddMesh function add all faces from one mesh to another
Also tweak type signature of trixApply to accept const mesh. Realized that `const trix_mesh *` only means the trix_mesh pointer itself is immutable; the content (faces, etc) can still be modified. (Duh.) This probably has consequences for types used throughout the rest of the lib.
trixRelease now takes a trix_mesh** like Create and Read, so that it can explicitly set the released trix_mesh* to NULL
Update sample tools for recent changes
Tweaked function declarations so mesh parameter is always first
Update sample tools for API tweaks