Under wxGTK+2 bitmap data with mask and without it (raw) should be stored
in the separate GdkPixbuf buffers - just like it's done in wxGTK+3. These
two buffers are necessary because only GdkPixbuf with raw bitmap data
(original, non-masked) should be copied when wxBitmapRefData instance is
cloned e.g. in SetMask(). GdkPixbuf with masked data is not copied and is
created on first use in wxBitmap::GetPixbuf().
Closes#18508.
See #18498.
f7247086c2