GitLab

* Missing include for `dlib::loss_multiclass_log_per_pixel_::label_to_ignore`

I was trying to compile the examples and encountered this issue after moving `rgb_label_image_to_index_label_image` to cpp file. Headers should include all symbols they mention.

* Update pascal_voc_2012.h

Should use the official entrypoint for including dnn stuff.
Co-authored-by: Davis E. King <davis685@gmail.com>

* add more cuda capabilities

* Allow setting custom cuda capabilities

* improve default behavior

* rename to compute capabilities

* add progress information (current/total and percent)

* print a new line instead of overwritting with spaces

* check if target_val is an integer with std::trunc

* add function to compute string dimensions in pixels

* use custom struct as a return value, remove first and last params

* Update dlib/image_transforms/draw_abstract.h
Co-authored-by: Davis E. King <davis@dlib.net>

* Fix cannot compile iso only code (#579)

also fixing (#1742)

* Remove GUI dependency from fonts (#2273)

* Fix disable_duplicative_biases when the input is a skip layer

* fix template parameters

* try to make sure travis uses C++17

* fix unbound variable

* Update dlib/travis/build-and-test.sh
Co-authored-by: Davis E. King <davis@dlib.net>

* Fix serialize variant with C++17

* fix order of parameters

* added support for std::optional if using C++

* oops, bug fix + check if item already holds a type

* oops, another bug fix

* remove warnings about unused parameters
Co-authored-by: pf <pf@me>

* [SERIALIZATION] addes support for std::variant

* [SERIALIZATION] bug fix + added tests

* support immutable types

* put an immutable type in std::variant
Co-authored-by: pf <pf@me>

Do this by just making all layers have a .input_layer() method, which in
that context can be implemented in a simple manner.

Also fix this error from cmake 3.5.1:

```
CMake Error at CMakeLists.txt:62 (if):
  if given arguments:

    "CMAKE_COMPILER_IS_GNUCXX" "AND" "CMAKE_CXX_COMPILER_VERSION" "VERSION_LESS_EQUAL" "4.8.5"

  Unknown arguments specified
```

* do not attempt to build with gcc 4.8.5 or older

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