Fix missing documentation for several GDI functions.

The documentation for wxGetClientDisplayRect(), wxGetDisplayPPI(),
wxGetDisplaySize() and wxGetDisplaySizeMM() didn't appear in the output
because they were documented as "overloads" of the corresponding functions
without "Get" prefix inside an @addtogroup Doxygen macro and apparently this
is not supported.

Simply document them separately to fix this.
Add a unit test for wxImage::Scale() method.

Check that resizing the test horse image produces the same results in the
future as it does now, by saving the current results in files and verifying
that images resized directly and loaded from these files are the same.

See #15281.
Add benchmarks for wxImage methods.

Build a GUI version of the benchmark program too and use it to benchmark
wxImage loading methods for now (more will be added later).
Add wxImage::Scale() benchmarks.

Measure the time taken to rescale the image to a bigger or smaller size using
normal or high quality.

See #15281.