Sizing the images stored in the list should stick to the convention
adopted in the native wxImageList implemented in wxMSW.
Images stored in the list should have the sizes as it is declared for
the list even if provided bitmaps have different sizes.
In case of discrepancies their dimensions should be adjusted accordingly
(cropped or extended).
If image using bitmap and mask colour is added to nonmasked wxImageList we
need to convert effective mask to alpha channel values prior to adding the
image to the native list to preserve transparency.
Bitmaps stored in the list as a result of replacing existing ones should
conform to the same constraints as bitmaps directly added to the list.
These constraints are applied in the shared GetImageListBitmap() function
called both on adding and replacing the images.
