Symbian raster surface doesn't get an alpha channel when WA_TranslucentBackground is set

When using the openvg graphics system, with the fix for QT-4416 applied, the following occurs if WA_TranslucentBackground is set after the native window is created:
1. Create widget
2. Show widget - EGL window surface is created
3. Set WA_TranslucentBackground attribute
a) Appropriate RWindow functions are called to make the native window transparent
b) EGL surface is destroyed, and replaced with a raster surface with EColor16MA(P) display mode

When using the raster graphics system:
1. Create widget
2. Show widget - raster surface is created with EColor16MU display mode
3. Set WA_TranslucentBackground attribute
a) Appropriate RWindow functions are called to make the native window transparent
b) Raster surface is left unchanged

The result is that the alpha values written by the raster paint engine into the raster surface are ignored by the window system. The EColor16MU bitmap needs to be destroyed and replaced with an EColor16MA(P) one.