Merge branch 'art-scalefactor'

Fix confusion between different kinds of coordinates in wxAUI code,
restoring correct behaviour in high DPI on all platforms.

See #2620.

Closes #19331.

docs/doxygen/mainpages/const_cpp.h

@@ -167,15 +167,19 @@ Currently the following symbols exist:
     have an efficient (CPU-specific) implementation. Notice that the functions
     themselves are always available but can be prohibitively slow to use when
     implemented in a generic way, using a critical section.}
-@itemdef{wxHAS_BITMAP_SCALE_FACTOR, Defined in @c wx/bitmap.h if bitmaps
-    actually use scale factor under the current platform, see
-    wxBitmap::SetScaleFactor().}
 @itemdef{wxHAS_BITMAPTOGGLEBUTTON, Defined in @c wx/tglbtn.h if
     wxBitmapToggleButton class is available in addition to wxToggleButton.}
 @itemdef{wxHAS_CONFIG_TEMPLATE_RW, Defined if the currently used compiler
     supports template Read() and Write() methods in wxConfig.}
 @itemdef{wxHAS_DEPRECATED_ATTR, Defined if C++14 @c [[deprecated]] attribute is
     supported (this symbol only exists in wxWidgets 3.1.6 or later).}
+@itemdef{wxHAS_DPI_INDEPENDENT_PIXELS, Defined if pixel coordinates on the
+    current platform scale with DPI, i.e. if the given length in pixels has the
+    same apparent size on the display independently of the DPI (this symbol
+    only exists in wxWidgets 3.1.6 or later). Note that it should rarely, if
+    ever, be necessary to use this symbol directly, functions such as
+    wxWindow::FromDIP() and wxBitmap::GetScaledSize() exist to hide the
+    differences between the platforms with and without DPI-independent pixels.}
 @itemdef{wxHAS_MEMBER_DEFAULT, Defined if the currently used compiler supports
     C++11 @c =default.}
 @itemdef{wxHAS_LARGE_FILES, Defined if wxFile supports files more than 4GB in