Keep missing texture names, plus added flag in Set Texture for texture substitution.

[Sallo97]

Same pull-request proposed in #1627 and #1628 , only on the devel branch.

Avoid deleting original texture names when loading

When loading a mesh, if meshlab was not able to find the associated texture files, it uses a dummy texture instead, named dummy.png. However, in doing so it also removes any mention to the absent texture(s). I modified the loading mesh process in src/common/ml_document/mesh_model.cpp to keep its missing texture name(s) list by commenting line 124 in src/common/ml_document/mesh_model.cpp:

// textName = "dummy.png";

Adding possibility of not erasing all the names in the Set Texture

When setting a new texture to a loaded mesh with the filter Set Texture, it clears all information regarding previous associated ones. This can be frustrating when meshlab loaded a mesh with missing texture, for which the user wants to manually load them up.

My solution consists in adding the boolean parameter updateExistingTexture to filter_texture s.t., each time a texture is loaded and the flag activated, then the filter does not clear any previously loaded mesh and instead if a texture with the same name of the new loaded file exists, its content is substitued with the one provided by the user.

[alemuntoni]

Thanks for the PR!
There are some build issues on macos due probably to some dependencies that must be updated. In any case, they aren't caused by your edits. I'll work and fix them ASAP.

[alemuntoni]

Thanks @Sallo97!