Lazy conversion in fromSymbianCFbsBitmap for OpenVG

For most CFbsBitmap-based use cases forcing RGB32 or ARGB32_Premultiplied for the image data behind QVGPixmapData is an overkill. The most common operation there is to draw the pixmap on the screen, and that can be done without any format conversion since many CFbsBitmap display modes map directly to Qt / OpenVG equivalents (e.g. EColor64K -> QImage::Format_RGB16 / VG_sRGB_565). Some operations, like painting into such pixmaps will suffer, but the frequency of these operations for pixmaps created from CFbsBitmap should be very low.