Fix #2228 - PImageResizeTransparantBug

This pull request fixes issue #2228.

**Problem:**
1. The resize() method creates a BufferedImage with the shrinkImage() method
2. The type set for opaque images is BufferedImage.TYPE_INT_RGB
3. The resize() method then creates a PImage from the BufferedImage with the PImage(Image img) constructor
4. The PImage(Image img) constructor uses raster.getDataElements(0, 0, width, height, pixels).
**5. When the type is BufferedImage.TYPE_INT_RGB the raster.getDataElements() method returns pixels with no transparency (aka alpha 00 instead of FF).**
5. The end result is that all fully opaque images ironically become fully transparent images. Hence they are no longer visibly drawn.

**Solution:**
Make sure that all BufferedImages created with the shrinkImage() method are of type BufferedImage.TYPE_INT_ARGB. With this change, the raster.getDataElements() method will return the correct alpha value for both opaque and transparent images.

**Consequences:**
The format variable is switched to ARGB via the PImage(Image img) constructor, since it follows the type of the BufferedImage.