Blender Git Loki

Remove axis transform defines and use existing enums

Clean up: comments, minor refactor.

Use `const char *set_name` instead of `void get_name` which was meant
to copy the buffer originally but then was neglected.

Don't expose `_export_mesh_eval` just for ensuring normals, moved it to
another function.

Use conventional destructor function names.

Upper case enums.

Check if there is any material to write.

Null check: no image is present in the texture node

Fix crash when exporting uv vertices with no UV map.

Support multiple materials in the same object.

Design has slightly changed:
The line with `usemtl` has moved just before a face element is
being written. So whenever a face with a new material is encountered,
it can be labelled.

Appending to the MTL file is still at the same place, writing once every
object.

Support material groups: writing material name with object name

Support for object groups: name of object and its mesh

Fix socket vector being read as socket RGBA

Wrong typecasting as bNodeSocketRGBA instead of bNodeSocketVector
causes `SOCK_VECTOR`'s `type` being read as `value[0]`.

So instead of having three different functions for float, vector & color, merging
into one and specifying Datatype seems better.

Use findNodeSocket since it accepts const bNode*

{rB340130719f4d}

Merge branch 'master' into soc-2020-io-performance

Add object name in MTL related errors.

Use Span<T> instead of const Vector<T> &

Reasons to prefer `Span`:
> - It is shorter.
> - It is const by default (whereas you could easily forget the const in `const Vector<T> &`).
And I did forget the `const` here!
https://developer.blender.org/D7987#inline-64852

Typo: write OBJ & MTL comments properly with #

Support exporting selected objects only, not the whole scene.

UI: show warning when overwriting an export file

Replace char * with string in texture map types.

Review update rB827869a45bccbf1e016580b#271046 (by Jacques Lucke)

As mentioned in his comments, `Map` will do a pointer comparison for
`char *` keys. So string would be better. Even though the `lookup` has
been removed, `std::string` is still okay.

Also instead of iterating over `Map.keys()`, and then looking up value,
now it's iterating over a key-value pair: `Map<T, T>::item` of strings.

Minor comment changes.

Remove redundant blender:: from Vector, Map etc.

In rB96d6571073d3, `namespace blender` was added.

Add texture transform options, Blender version comment.

Two texture transform options, out of several, are supported now:
- Translation (origin offset) (syntax: "-o u v w")
- Scale (syntax: "-s u v w")

For Normal Map textures, Strength (bump multiplier) (syntax: "-bm s")
is also added.

I had to temporarily replace `nodeFindSocket` with its own
implementation to be able to build, since it doesn't accept
`const bNode*` which we're using everywhere.

I proposed a simple fix: D8142 for that. If and when it gets committed,
I'll remove the duplicate code.

Blender version string is also added in the MTL file.

Test file: {F8647881}

Move code to namespace blender; clang-format