alpha values mistakenly applied twice in DrawImage.renderImageXform()

In some cases, it is possible to get incorrect blending results (alpha values
appear to be applied twice) when transforming an image with JDK 6 or higher.
This only occurs for certain destination types where a native MaskBlit loop is
not implemented (e.g. OpenGLSurface, a TYPE_FLOAT BufferedImage, and so on).